Does Smoking Make Crohn's Worse? The Evidence

Yes. Of everything you can personally change about the course of Crohn's disease, smoking has the largest and most consistent evidence behind it. Pooled across 33 observational studies, smokers with Crohn's had roughly 56% higher odds of a flare, 68% higher odds of needing a first operation and more than double the odds of needing a second one (To 2016).

And yet this is one of the worst-explained facts in inflammatory bowel disease, for one specific reason. The other main form of IBD, ulcerative colitis, has a smoking story that runs the opposite way, and the two keep getting mixed up. Google's own autocomplete will finish "smoking crohn" with "is smoking protective for crohn's" and "smoking helps crohn's". In the ulcerative colitis threads, people with Crohn's turn up regularly to correct that, and the confusion still circulates. The colitis side of that story, including the nicotine trials, has its own guide: does smoking help ulcerative colitis.

So this guide does three things. It gives you the actual numbers with their uncertainty, rather than the vague "smoking is bad for you" that most pages settle for. It separates the Crohn's evidence from the ulcerative colitis evidence, and shows that even the colitis version is shakier than its reputation. And it deals honestly with the three questions the threads keep asking that nobody answers: what quitting changes once you are already diagnosed, whether vaping and nicotine pouches count, and where cannabis fits, which turns out to be a completely different question.

The short answer, with the numbers and their limits

The single best summary is a 2016 systematic review and meta-analysis that pulled together 33 observational studies on smoking and the course of Crohn's disease. Compared with non-smokers, smokers had:

Every one of those confidence intervals sits well clear of 1, which is the no-difference line, so these are not borderline findings (To 2016). The same analysis reported that the odds among ex-smokers fell back to levels comparable with non-smokers, and for flare and second surgery were significantly lower than in current smokers. A funding note, since it costs one clause: that review's listed support is a CORE bursary, and its companion paper on ulcerative colitis, discussed below, lists a Dr Falk CORE bursary, which names a pharmaceutical company in the funding of the colitis analysis. We did not read either paper's conflict of interest statement, so we are not characterising the authors' own ties either way.

Three things those numbers are not, and it matters that you know which is which.

They are not randomised. You cannot randomise people to smoke. Every figure in that table comes from observing people who chose, or were driven by addiction, to smoke. Smoking travels with other things: deprivation, stress, poorer access to care, other substance use. Good studies adjust for what they measured, and none of them adjusted for everything.

They are not about how bad your symptoms feel. They are about flares, operations and treatment escalation. That is arguably a higher bar than a symptom score, which is worth knowing given how loosely the word "flare" gets used. Our guide to how to tell if you are in a Crohn's flare covers why a bad symptom week and objectively active inflammation are not the same thing.

They are not the multiples you may have read. Some general health pages state that smokers are two to five times more likely to develop Crohn's. The strongest sources on development are more modest. A meta-analysis found current smoking associated with Crohn's at an odds ratio of 1.76 (1.40 to 2.22) (Mahid 2006), and the largest prospective cohort, 229,111 women in the Nurses' Health Studies, found a multivariate hazard ratio of 1.90 (1.42 to 2.53) in current smokers and 1.35 (1.05 to 1.73) in former smokers, with risk rising as pack-years rose (Higuchi 2012). Note that population: US female nurses, mostly white, so the estimate may not transfer cleanly. Roughly twofold is the defensible number. Fivefold is not.

Why you keep reading that smoking helps, and why that is a different disease

The protective-looking signal is real, it is specific to ulcerative colitis, and it runs in the opposite direction in Crohn's. The same meta-analysis that found current smoking associated with Crohn's at an odds ratio of 1.76 found current smoking associated with lower odds of developing ulcerative colitis, at 0.58 (0.45 to 0.75), and former smoking associated with higher odds of colitis at 1.79 (1.37 to 2.34) (Mahid 2006). The Nurses' Health Study data show the same split: current smoking raised Crohn's risk but not colitis risk, while former smoking raised colitis risk and that elevation persisted for more than two decades after quitting (Higuchi 2012).

An umbrella review of 53 meta-analyses covering 71 environmental factors graded the evidence for smoking increasing Crohn's risk as high to moderate strength, putting it in a short list of nine factors that clear that bar (Piovani 2019).

Here is the part almost nobody tells colitis patients, and it matters for Crohn's patients too because it explains why the folklore is stickier than the evidence. The same team that produced the Crohn's meta-analysis produced a companion one for ulcerative colitis: 16 studies covering colectomy in 2,615 patients, flare in 620, proximal disease extension in 687 and pouchitis in 355. Compared with non-smokers, smokers' odds of colectomy (0.89, 0.62 to 1.26), flare (1.26, 0.65 to 2.44), proximal extension (0.57, 0.20 to 1.66) and pouchitis (0.57, 0.21 to 1.53) were not significantly better. Their conclusion was that smoking may not improve the natural history of ulcerative colitis at all, and that cessation advice belongs in colitis guidance too (To 2016b).

So the honest summary of the paradox is this. Smoking looks protective against getting ulcerative colitis, and stopping looks like a trigger for its onset. Whether it improves colitis once you have it is much less certain than the forums assume. And in Crohn's disease the direction is reversed on both counts.

In the r/UlcerativeColitis thread asking for "the story of smoking and UC", replies described quitting being followed by a first flare, and people restarting a couple of cigarettes a day and finding symptoms settle (r/UlcerativeColitis thread). Those experiences are not nothing. The study behind them is small: 32 people with colitis who stopped smoking, compared with 32 non-smokers and 32 continuing smokers matched for sex, age and age at onset, and it did report more active disease and more need for hospital admission and major therapy after quitting (Beaugerie 2001). Thirty-two people is a sample you would not build a treatment plan on, and the British Society of Gastroenterology grades its recommendation on smoking in ulcerative colitis as weak, on very low quality evidence, while still recommending cessation and advising that a plan be made to mitigate the flare risk (Lamb 2019).

For the Crohn's recommendation, the same guideline uses a strong recommendation with 100% panel agreement, and a separate strong recommendation, on moderate quality evidence, that anyone smoking after intestinal resection for Crohn's be actively encouraged to stop.

If your diagnosis has shifted or is unclear, this distinction is genuinely consequential and worth resolving with your team rather than with a search engine. One r/IBD post described exactly this bind: a proctitis finding, a biopsy saying only "inflammatory bowel disease", and a person trying to work out from how smoking made them feel whether they had Crohn's or colitis (r/IBD thread). Your reaction to a cigarette is not a diagnostic test, and nothing in the literature supports using it as one.

Does the amount matter?

Less than most people hope, and the evidence here is genuinely a little untidy.

A 1999 series of 622 people with Crohn's in clinical remission found 46% of current smokers (139 people) had a flare during follow-up, versus 30% of non-smokers (79) and 23% of former smokers (13). The adjusted relative risk of flare in current smokers was 1.35 (1.03 to 1.76). The authors reported that this risk became statistically significant above a threshold of 15 cigarettes a day, and that former smokers behaved like non-smokers (Cosnes 1999).

A decade later the same Paris group looked specifically at light consumption across 2,795 consecutive patients, splitting them into non-smokers (1,420), light smokers of one to ten a day (385), heavy smokers of more than ten (638) and intermittent smokers (352). Light smokers spent 46% of patient-years with active disease against 37% for non-smokers, and needed more immunosuppressants. Their conclusion was that light smokers do worse than non-smokers and that complete cessation should be advised in all smokers with Crohn's (Seksik 2009).

We could not fully reconcile those two. The first says the flare-risk increase only reaches significance above 15 a day; the second, larger and using person-years rather than individuals, finds a clear penalty at one to ten a day. The likeliest explanation is statistical power and a different outcome measure rather than a genuine contradiction, but we cannot settle it from the abstracts. The British Society of Gastroenterology reads the field as concluding that passive smoking and light smoking of fewer than ten a day are just as bad as heavy smoking (Lamb 2019), and grades the underlying evidence for its Crohn's cessation statement as low quality while still making the recommendation strong.

A separate retrospective series of 1,115 people with Crohn's across six Australian and New Zealand sites, followed for a mean of 16.6 years, reported that smoking more than three cigarettes a day was associated with increased risk of developing stricturing or penetrating disease, at an odds ratio of 3.8 (1.27 to 11.17) (Lawrance 2013). Read that confidence interval carefully: it stretches from a modest effect to an enormous one, which is what happens when a subgroup is small. The direction is informative; the precise multiple is not.

Why women with Crohn's appear to fare worse

A study of 1,784 people with colitis, comprising 978 with ulcerative colitis, 118 with indeterminate colitis and 688 with Crohn's colitis, set out to look at the effect of smoking on both diseases in parallel and to test whether sex modified it. Ever-smoking was more common in Crohn's colitis (61%) than ulcerative colitis (42%). Smoking cessation preceded disease onset in 61% of the ulcerative and indeterminate colitis group but only 12% of the Crohn's colitis group. The authors concluded that the dual effects of smoking in colitis, apparently beneficial in ulcerative colitis and harmful in Crohn's colitis, were importantly modulated by sex, with women at more disadvantage than men (Cosnes 2004). The British Society of Gastroenterology states this as established: the adverse effects of smoking are more pronounced in women than in men with Crohn's disease (Lamb 2019).

Swiss IBD Cohort data on 1,770 patients found 29% were current smokers overall, and reported smoking rates in Crohn's stayed persistently and stagnantly elevated compared with an age, sex and year-matched general population, especially in women. One detail worth flagging: the published abstract writes "UC, 39.6% vs CD 15.3%" in a sentence claiming more than twice as many Crohn's patients smoke, which is internally inconsistent. Read against the paper's own conclusion that smoking rates in Crohn's are alarmingly high, the labels appear transposed and 39.6% is almost certainly the Crohn's figure. We flag it rather than quietly picking a reading. The paper's substantive finding, which does not depend on which way round those two numbers go, is an unacceptable underuse of supportive measures to help people stop (Biedermann 2015).

If your Crohn's symptoms already swing with your cycle, that is a separate and real pattern; we cover the evidence in why your Crohn's is worse on your period.

What smoking appears to be doing inside a Crohn's gut

Nobody has a complete mechanism, and you should be suspicious of any page that presents one confidently. What exists is a set of partial findings that point the same way.

Immune cells behave differently in Crohn's smokers, and it does not look like a nicotine effect. Blood mononuclear cells from smokers with Crohn's secreted lower levels of chemokines and cytokines than cells from non-smoking counterparts, while healthy smokers and smokers with ulcerative colitis showed no such impairment. The same pattern appeared after exposing cells to cigarette smoke extract in the lab, and cells from Crohn's smokers showed defective protective responses, including lower levels of the cytoprotective protein Hsp70. The authors' reading was that the responses depended largely on oxidative stress generated by smoke, not on the nicotine component (Bergeron 2012). That distinction is worth holding on to when you get to the vaping section.

The gut bacterial community differs. In 103 people with active Crohn's and 66 healthy controls, smoking had a significant and independent effect on the microbiota of Crohn's patients, with higher Bacteroides-Prevotella in smokers (38.4%) than non-smokers (28.1%). Healthy smokers showed a similar shift (Benjamin 2012). This is an association from a cross-sectional study using a targeted probe method, not a demonstration that the bacteria cause anything, and the denominators in the results (101 patients, 58 controls) are smaller than the numbers recruited without the abstract explaining the drop.

There is a plausible T-cell link to post-surgical recurrence. In 57 patients in a prospective multicentre cohort, active smokers at the time of surgery had a significantly higher proportion of clonal T-cell expansions in the ileal mucosa than non-smokers (25.9% versus 17.9%). Every one of the 18 patients whose clonality exceeded 26.8% had endoscopic recurrence, and those high-clonality patients were much more often smokers (61% versus 23%) (Allez 2019). Fifty-seven people is small; the study was funded by the Helmsley Charitable Trust.

Smoking tracks with the more structurally damaging disease patterns. In 163 patients who had non-stricturing, non-penetrating disease at diagnosis, active smoking was associated with developing a penetrating pattern by five years. Be precise about what this study found: in multivariate analysis, only disease location and number of flares per year significantly discriminated between the three behaviour groups, and smoking was associated with penetrating disease only in the comparison against non-stricturing, non-penetrating disease (Louis 2003). That is a narrower claim than "smoking causes strictures and fistulas", which is how the same paper often gets quoted.

Does quitting change anything once you already have Crohn's?

The evidence says former smokers look much more like people who never smoked than like current smokers. That is the consistent finding. It is also weaker evidence than the strength of the guideline recommendations might suggest, and it is worth being clear why.

The study everyone cites is a French intervention study. Repeated counselling plus easy access to a cessation programme was offered to 474 consecutive smokers with Crohn's disease. Fifty-nine (12%) stopped for more than a year, and those quitters were followed prospectively against two matched control groups, continuing smokers and non-smokers, over a median 29 months. The risk of flare in quitters did not differ from non-smokers and was lower than in continuing smokers. Need for steroids and for introducing or reinforcing immunosuppressive therapy was similar in quitters and non-smokers, and increased in continuing smokers (Cosnes 2001).

Two things about that paper deserve saying out loud.

First, the risk of surgery was not significantly different across the three groups. That is a null result inside the study most often used to argue that quitting improves the course of Crohn's, and it does not usually get mentioned.

Second, the abstract contradicts itself. The results section states that flare risk in quitters did not differ from non-smokers. The conclusion states that patients who stop for more than a year have "a more benign disease course than if they had never smoked", which is a stronger claim than the results support. We could not reconcile those two statements from the abstract. The pooled meta-analysis is the more careful version: ex-smokers' odds were comparable to non-smokers (To 2016). If you see a page claiming quitting makes Crohn's better than never smoking, that sentence is where it comes from, and it should not be quoted without the results paragraph beside it.

On surgery specifically, a meta-analysis of 16 studies and 2,962 patients found smokers had significantly higher clinical postoperative recurrence than non-smokers, at an odds ratio of 2.15 (1.42 to 3.27), and concluded a roughly 2.5-fold increased risk of surgical recurrence and twofold risk of clinical recurrence (Reese 2008). Its statement that risk is minimised on cessation rests on only 137 ex-smokers out of 2,962 participants, which is 4.6% of the sample. Take the smoker-versus-non-smoker comparison as solid and the ex-smoker comparison as thin.

And the honest limit on all of it: no randomised trial has tested whether a cessation intervention reduces disease activity in Crohn's disease. The nearest thing in the literature is a Cochrane review of cessation interventions in chronic autoimmune inflammatory joint diseases, which found only two studies, both in rheumatoid arthritis, neither showing benefit from the intervention tested, and neither even assessing or reporting disease activity. The certainty of evidence was rated very low (Roelsgaard 2019). That review is not about IBD and we are not applying its result to IBD. We cite it to make one point: the type of trial that would nail this down has not been done in Crohn's either.

What actually happens in the first weeks, and what nobody has measured

There is no published dataset tracking Crohn's symptoms week by week through a quit attempt. If a page gives you a confident timeline, it made it up. Here is what can be said.

Nicotine speeds gut transit and is a stimulant, so bowel habit commonly changes when it stops. Withdrawal itself affects sleep, concentration and mood for the first weeks. In the Crohn's threads, the reported changes are on a scale of months rather than days: fewer daily bowel movements, less pain, nausea becoming rare rather than routine (r/CrohnsDisease thread). One person quitting on the day of their first colonoscopy described three days of extra gas and heavy brain fog while preparing a thesis defence, and replies pointed out that much of the gas was likely residual air from the procedure and that stopping several stimulants at once takes time to settle (r/CrohnsDisease thread).

Treat all of that as lived experience, not evidence. It is useful for knowing you are not alone and useless for predicting your own course. Two specific traps:

Do not write off a genuinely new symptom as withdrawal. Fresh rectal bleeding, unexplained weight loss, fever, waking at night with symptoms or new severe pain deserve a conversation with your IBD team promptly, whatever else is going on in your week.

Do not expect symptom improvement to be the thing that keeps you quit. The benefits in the literature are about flares, operations and treatment escalation over years. If your nausea improves within weeks, that is a genuine win but not the mechanism the cohort studies measured. If it does not, that is not evidence quitting was pointless. Nausea has plenty of other explanations in Crohn's, which we go through in nausea after eating with Crohn's, and post-quit brain fog overlaps heavily with the Crohn's brain fog picture.

Relapse, and the support that exists

Relapse is the part that gets least airtime and probably deserves the most.

In a Spanish registry cohort of 121 people with IBD who had been active smokers at diagnosis and later became ex-smokers, relapse back to smoking was more common in Crohn's (46%) than ulcerative colitis (24%), with a median time to first relapse of 12 months in Crohn's. Nicotine dependence was the only independent factor associated with relapse (Gonzalez-Munoza 2024). Nearly half, at around a year. Plan for that rather than being ambushed by it.

The British Society of Gastroenterology puts unaided long-term abstinence at under 10%, and sets out what should happen instead: everyone with IBD asked about smoking, those who smoke advised of the harm and advised to stop, and offered referral to a cessation service where they are offered behavioural therapy in combination with pharmacotherapy (Lamb 2019). We are not going to tell you which aid to use or at what dose; that is a conversation for your GP, your IBD team or a cessation service, and their answer will depend on your history. What the general-population evidence shows is that the licensed nicotine replacement forms work: across 133 trials and 64,640 participants, the relative risk of abstinence at six months or more versus control was 1.55 (1.49 to 1.61), rated high-quality evidence, with the gum at 1.49, the patch at 1.64, tablets and lozenges at 1.52 (Hartmann-Boyce 2018). That evidence is in general smokers, not specifically in people with Crohn's.

What has been tried in IBD clinics specifically:

• Advice and counselling at scale. The TABACROHN study enrolled 408 Crohn's smokers, instructed everyone about the risks at inclusion, and followed them three-monthly for a median 18 months. Thirty-one per cent achieved complete cessation and 23% were smoking-free at the end of follow-up, with 8% relapsing. Strikingly, 88% tried to quit with no pharmacological therapy at all (Nunes 2013). There was no control group, so nobody knows how many of those 31% would have quit anyway; that is the number that would matter and the study cannot supply it.
• A dedicated pharmacist in the clinic. A small prospective programme referred 32 Crohn's smokers; 19 enrolled and 13 declined. Over a median 305 days, 42% (8 of 19) of enrolled patients quit at least once versus 15% (2 of 13) of those who declined. Do the subtraction: that is a 27 point gap, not a 42% effect, and the people who declined were self-selected, which is not a randomised comparison. Patients who kept smoking had more instances of losing response to a biologic, needing a new one, or escalating therapy (Tse 2022). Nineteen participants is a feasibility study. Several authors are prominent IBD trialists with extensive industry relationships across the field; we did not read this paper's conflict of interest statement, so we are not characterising it.
• The economics. A Canadian cost-utility analysis over a five-year Markov horizon compared no programme, counselling, nicotine replacement, nicotine replacement plus counselling, and varenicline for Crohn's. All four active strategies dominated no programme, meaning better quality-adjusted life years at lower cost. No programme only became more cost-effective if every strategy cost roughly ten times the estimate (Coward 2015). It was funded by the Canadian Institutes of Health Research. A model is a model, but the direction is a useful argument to have in hand if your service says funding is the obstacle.

And the awareness gap that makes all of this harder. In a London clinic survey of 246 IBD outpatients, only 52% of the 173 Crohn's patients knew that smoking worsens Crohn's, and knowledge was unrelated to smoking status. Nicotine dependence in these patients was lower than in people attending a smokers' clinic and comparable with the general population, which the authors read as meaning most could be helped to stop within the IBD clinic itself (Wahed 2011). A larger Belgian survey of 625 Crohn's and 238 colitis patients reached the same verdict: Crohn's patients were better informed than colitis patients, and the awareness rate was still low (De Bie 2015).

Does vaping count? What about nicotine pouches and snus?

The honest answer is that nobody knows, and the gap between how confidently this gets discussed and how little has been measured is large.

E-cigarettes. The only clinical outcome study we could find is a retrospective case-control study of 127 people with IBD who were current users of nicotine-containing e-cigarettes, matched to 251 non-vaping controls. The outcome was a composite of starting a biologic, switching biologic, or IBD-related hospitalisation or surgery over two years. In Crohn's, current e-cigarette use was not associated with a higher risk (odds ratio 0.82, 0.36 to 1.87); in ulcerative colitis, also not (1.05, 0.33 to 3.39). The authors' own framing is the right one: this does not show vaping is safe, larger studies of people newly taking up vaping are needed, and patients should be discouraged from starting given the overall health effects (Sheehan 2023). Look at that first confidence interval: it runs from a 64% reduction to an 87% increase. It is consistent with meaningful harm.

A 2025 laboratory study compared immune cells from people with ulcerative colitis using combustible cigarettes, using electronic nicotine delivery systems, or not smoking, alongside a chemically induced colitis model in mice, and reported that both exposures attenuated inflammation and enhanced mucosal healing through different mechanisms (Kastratovic 2025). Three things to hold on to. That study is in ulcerative colitis, not Crohn's. Much of it is mouse work with chemically induced colitis, which is not Crohn's disease. And the Crohn's mechanism work discussed earlier suggested the harmful effect there ran through oxidative stress from smoke rather than through nicotine (Bergeron 2012), which means a nicotine-only product is not obviously equivalent in either direction. Do not read a colitis mouse result as reassurance about Crohn's.

The British Society of Gastroenterology noted in 2019 that there was very little evidence on e-cigarettes in ulcerative colitis, and that a case report surprisingly showed no benefit despite comparable nicotine delivery (Lamb 2019).

Nicotine pouches and snus. We searched for any study of nicotine pouches or snus in Crohn's disease or IBD and found none. Zero. That is worth stating because pouches come up repeatedly in the threads, including from people who switched from cigarettes to pouches and felt better, which is entirely compatible with the improvement coming from stopping smoke rather than from the pouches. An updated public health scoping review of 73 new studies on oral nicotine pouches (12 industry-funded) found sales more than tripled in the US between 2021 and 2024, that pouches deliver more nicotine than cigarettes at higher strengths, that preliminary short-term evidence suggests they can reduce smoking but generally produce dual use rather than complete cessation, and that they are considerably less toxic than cigarettes with long-term effects still understudied (Travis 2026). None of that is about your gut.

Nicotine as a treatment. For completeness, because it comes up: nicotine has actually been trialled in ulcerative colitis, and it did not go well. A Cochrane review of transdermal nicotine for inducing remission in ulcerative colitis found five eligible trials (McGrath 2004), and a later meta-analysis of three placebo-controlled trials in 233 patients plus two comparing nicotine with corticosteroids in 81 patients found a summary relative risk for clinical remission of 1.40 (0.63 to 3.12), which crosses the no-effect line, and concluded the findings did not support either the efficacy or the tolerability of nicotine preparations for inducing remission (Nikfar 2010). That abstract is itself a little confused, describing maintenance of remission in its background and induction in its objectives and title; we read the conclusion as applying to induction, as the title states. There is no equivalent literature suggesting nicotine treats Crohn's, and no reason from the mechanism work to expect one.

Cannabis: a separate question with its own thin evidence

In r/CrohnsDisease, cannabis threads dwarf tobacco threads, with multiple discussions running past a hundred comments. It is a genuinely different question from tobacco and deserves to be treated as one rather than folded into "smoking".

A Cochrane review found three small studies, 93 participants in total, all in active Crohn's, all rated very low certainty, none in quiescent disease. Two enrolled adults who had failed at least one medical treatment. The studies were not pooled because the interventions differed (Kafil 2018).

The trial you will see quoted is the largest of the three: 21 people with a Crohn's Disease Activity Index above 200 who had not responded to steroids, immunomodulators or anti-TNF therapy, randomised to eight weeks of cannabis cigarettes containing 115 mg of THC or to placebo cigarettes made from cannabis flowers with the THC extracted (Naftali 2013).

This trial is a good case study in why you read the whole abstract. Its results paragraph describes "complete remission" in 10 of 11 in the cannabis group (90%) against 4 of 10 on placebo (40%), p = 0.028, while its own conclusion states that the primary endpoint, induction of remission, was not achieved. Those two sentences cannot both be right as written. Cochrane's independent extraction resolves it: clinical remission was 5 of 11 (45%) versus 1 of 10 (10%), relative risk 4.55 with a confidence interval of 0.63 to 32.56, which includes no effect; clinical response, defined as a fall of more than 100 CDAI points, was 10 of 11 (91%) versus 4 of 10 (40%), relative risk 2.27 (1.04 to 4.97). So the 90% figure is the response rate, not remission, and the trial did miss its primary endpoint exactly as its own conclusion says (Kafil 2018).

Two more things from that same extraction. The placebo group's 40% response rate is high, so the intervention-specific gain on response is about 51 points rather than 91. A responsive control arm is a sign of a properly constructed placebo, not a flaw. And adverse events were more common with cannabis cigarettes, at a relative risk of 4.09 (1.15 to 14.57), described as mild: sleepiness, nausea, difficulty concentrating, memory loss, confusion and dizziness. Cochrane rated all of this very low certainty, and the trial was rated high risk of bias for blinding, which is unsurprising given that one group was smoking THC.

The counterweight is a cross-sectional questionnaire of 313 consecutive IBD patients at a Canadian centre. Cannabis had been used specifically to relieve IBD symptoms by 17.6%, overwhelmingly by inhalation (96.4%). Reported benefits were substantial: improved abdominal pain in 83.9%, cramping in 76.8%, joint pain in 48.2%, diarrhoea in 28.6%. But using cannabis for more than six months at any time for IBD symptoms was a strong predictor of requiring surgery in Crohn's disease, with an odds ratio of 5.03 (1.45 to 17.46) after adjusting for demographics, tobacco status, time since diagnosis and biologic use. It did not predict hospitalisation (Storr 2014). That confidence interval is very wide, this is self-reported questionnaire data at a single point in time, and reverse causation is highly plausible: people with worse disease heading for surgery have more pain and more reason to use cannabis. The authors' conclusion, that patients should be cautioned about potential harm until trials evaluate efficacy and safety, is appropriately hedged.

More recently, a retrospective case-control study matched 62 IBD patients reporting cannabis use to 62 who did not, among people having endoscopy for acute IBD-related complaints. Cannabis users had longer endoscopic durations and were more likely to show inflammation endoscopically. Notably, there was no significant difference in stricture rates, in IBD treatment at the time of endoscopy, or in propofol dose once adjusted for procedure duration (Loeb 2025). A hundred and twenty-four people at one centre, and a design that cannot tell you which came first.

The summary: cannabis appears to relieve symptoms in Crohn's for a substantial number of people who use it, there is no good evidence it reduces inflammation, the one placebo-controlled trial missed its primary endpoint, and two observational datasets contain signals pointing towards worse disease among users that could equally reflect worse disease driving use. If you use cannabis for symptoms, the practical point is to tell your IBD team, both because it affects sedation planning for procedures and because symptom relief can mask inflammation that still needs treating.

Does smoking blunt your Crohn's medication?

The claim you will see is that smoking makes biologics less effective. For the start of infliximab, the pooled evidence says no.

A meta-analysis identified 12 articles, excluded four for using non-validated scoring systems, and pooled eight studies covering 1,658 patients of whom 649 were active smokers. The relative risk of responding to infliximab induction among smokers was 0.99 (0.88 to 1.11). Restricting to the five studies covering both inflammatory and fistulising disease gave 0.92 (0.80 to 1.06). Both include 1.0 comfortably. The authors were careful: though smoking worsens Crohn's, this analysis does not show a negative effect of smoking on initial response to infliximab, and long-term maintenance of response may yet be influenced by smoking status (Inamdar 2015).

Report that as a negative, because it is one. The Crohn's threads carry the belief that smoking makes infliximab less effective, and the pooled induction data do not support it.

What the cohort data do show is escalation over time. Continuing smokers in the French intervention study needed more steroids and more introduction or reinforcement of immunosuppressive therapy than quitters or non-smokers (Cosnes 2001). The small pharmacist-led programme observed more loss of response, switching and dose escalation among those who kept smoking (Tse 2022). Neither is a randomised comparison. The defensible statement is that smoking is associated with needing more treatment over years, and is not associated with failing to respond to infliximab at the start.

A worked example: what tracking around a quit attempt can actually show

Say you stop on a Monday. Here is what a realistic four-week record looks like, and what it can and cannot tell you.

Week one. Bowel movements per day jump, then fall. Sleep is broken. Concentration is poor. Appetite climbs. One bad cramping day. What this tells you: almost nothing about Crohn's. Nicotine withdrawal plus a change in gut transit explains all of it. Do not draw conclusions.

Week two. Bowel movements settle lower than your pre-quit average on four of seven days. Two days with more bloating. Mood low midweek. What this tells you: still nothing conclusive, but you now have a baseline forming.

Week three. Daily count stabilises. The post-coffee urgency you always attributed to coffee is unchanged, which is useful information in itself. One evening of pain after a large meal. What this tells you: you are starting to separate what was the cigarette from what was always there.

Week four. A fortnight of stool consistency records that look different from your pre-quit fortnight, plus a note that you have not needed the anti-diarrhoeal you kept in your bag. What this tells you: something worth showing your IBD team at your next appointment, alongside the date you stopped.

What this record cannot do is tell you whether inflammation changed. That needs a faecal calprotectin, blood tests or a scope, ordered by your team. A symptom log is evidence of your experience and a prompt for the right test; it is not a substitute for one.

The reason to write it down rather than remember it is that four weeks into a quit attempt your memory of week one is unreliable, and because "it is a bit better I think" gets far less traction in a ten-minute appointment than a dated record. This is the kind of thing Clairop is built for: logging that takes seconds so it survives a hard week, and a summary you can take to an appointment. It does not diagnose anything and it is not a substitute for your IBD team.

If you want the mechanics of keeping a record that holds up, how to keep a food diary covers the in-the-moment versus bedtime logging problem and why informal trigger hunting throws up false positives.

When to see a doctor promptly

Quitting smoking does not change any of this. Contact your IBD team or a doctor promptly if you have:

• New or increasing blood in your stool, or blood mixed through it rather than on the surface
• Unexplained weight loss
• Fever, or night sweats
• Symptoms that wake you from sleep
• Cramping waves of pain with vomiting, a swollen abdomen, and no stool or wind, which needs urgent same-day assessment for obstruction
• Symptoms that are worse and staying worse for more than a few days, rather than settling
• Signs of anaemia: breathlessness on stairs, unusual fatigue, palpitations, looking pale
• New perianal pain, swelling or discharge

If you are starting a quit attempt and you have Crohn's, it is worth telling your IBD team beforehand rather than afterwards, so a change in symptoms has context and so cessation support can be arranged properly. The guideline expectation is that they ask and offer. Our list of questions to ask your doctor about Crohn's includes the smoking conversation, and how to prepare for a GI appointment covers getting the most out of a short slot.

Myths, including the ones that point the other way

"Smoking helps IBD." It is specific to ulcerative colitis, and in Crohn's the association runs the other way (Mahid 2006). In colitis itself, a meta-analysis of 16 studies found no significant benefit on colectomy, flare, disease extension or pouchitis (To 2016b).

"It's too late for me, I've already had surgery." The retrospective series that asked exactly that question concluded all Crohn's patients should be encouraged to stop regardless of when diagnosed or how many operations they have had (Lawrance 2013), and the British Society of Gastroenterology makes a strong recommendation on moderate-quality evidence that people smoking after intestinal resection be actively encouraged to stop (Lamb 2019).

"A couple a day is fine." Light smokers of one to ten a day spent 46% of years with active disease against 37% for non-smokers, and needed more immunosuppressants (Seksik 2009). An earlier series found significance only above 15 a day (Cosnes 1999), so the exact dose threshold is unsettled, but "a couple is fine" is not what the larger dataset shows.

"Quitting makes Crohn's better than never smoking." This one comes from one paper's conclusion sentence, which contradicts its own results paragraph (Cosnes 2001). The pooled version says ex-smokers are comparable to non-smokers (To 2016).

"Smoking makes infliximab stop working." Not at induction. Eight studies, 1,658 patients, relative risk of response 0.99 (0.88 to 1.11) (Inamdar 2015). Long-term maintenance was not settled by that analysis.

"Quitting smoking will fix my Crohn's." No. The cohort data are about lower odds of flares, operations and escalation. In the study most cited for the benefits of quitting, surgery risk was not significantly different across quitters, continuing smokers and non-smokers (Cosnes 2001), and no randomised trial has tested cessation as a treatment in Crohn's at all (Roelsgaard 2019).

"Vaping is the safe version for Crohn's." Unknown. One case-control study of 127 vapers found no association with worse outcomes, with a confidence interval wide enough to include real harm, and its own authors advised against starting (Sheehan 2023).

"The link is proven, it's causal, end of story." Not quite. A Mendelian randomisation study found no clear genetic evidence that smoking causes Crohn's (Georgiou 2021). It examined disease onset rather than disease course, and genetic proxies for smoking behaviour are imperfect, but it is the strongest published challenge to the causal reading and it belongs in an honest account.

The bottom line

If you have Crohn's disease and you smoke, the association with a harder disease course is as well established as anything in this field that has not been randomised: higher odds of flares, of a first operation, of a flare after that operation, and of a second operation, with confidence intervals well clear of no effect. Light smoking does not appear to be a safe compromise, and the adverse effect looks larger in women.

What the evidence does not support is a promise. Ex-smokers in these cohorts look like people who never smoked, not better than them. Surgery risk was not significantly different between quitters and the comparison groups in the study usually cited for the benefits of quitting. No randomised trial has tested cessation as a treatment for Crohn's disease. And the strongest genetic study found no clear causal link to disease onset, which is a different question from course but still a finding that belongs on the table.

Nobody knows whether vaping, pouches or snus carry the same risk, because almost nobody has looked. Cannabis is a separate question where symptom relief looks real, anti-inflammatory benefit does not, and two observational signals point towards worse disease among long-term users without being able to establish which way the arrow runs.

The clearest practical gap is not evidential, it is communication. Only about half of Crohn's patients in one clinic survey knew smoking worsens their disease, nicotine dependence in IBD clinics is no higher than in the general population, most people in the largest cessation study tried without any medication, and cohort data describe supportive measures as underused. If you smoke and you have Crohn's, the conversation to have is with your IBD team or a cessation service, and the guidelines say they should be starting it.