B12 Deficiency After Ileostomy: Who Is at Risk

An ileostomy on its own rarely causes B12 deficiency. What causes it is losing, or damaging, the last stretch of small intestine where B12 is absorbed: the terminal ileum. A standard colectomy with an end ileostomy usually leaves most of that in place, which is why one hospital study of 39 people with an end ileostomy for inflammatory bowel disease, followed for a mean of 12.5 years, found only two who were deficient (Jayaprakash 2004). The people who do run into trouble are those who also had ileum removed, usually for Crohn's disease, and some people with pouches.

That distinction is missing from most of what you will read. Many stoma-charity and supplier pages say an ileostomy puts you at risk because "the part that absorbs B12 may be removed", and the search snippet for one forum post near the top of the results calls deficiency a matter of when, not if. Neither matches the studies. This article sorts out who is really at risk, how much ileum has to go before it matters, which blood test to trust, and how often checking makes sense. It does not tell you what to take: that is a decision for your team, and the evidence on it is weaker than it should be.

Does an ileostomy cause B12 deficiency?

Not usually by itself. An ileostomy diverts the end of the small intestine through the abdominal wall, but B12 is absorbed in the terminal ileum, which a standard colectomy mostly keeps. Deficiency follows when extra ileum is removed or stays inflamed, which is common in Crohn's and rare after colectomy for ulcerative colitis.

The cleanest test of this question is a study from a UK teaching hospital that identified every patient with an end ileostomy for IBD formed more than 30 months earlier: 18 with Crohn's disease, 17 with ulcerative colitis and 4 with indeterminate colitis, 39 in total. Their ileostomies had been in place for a mean of 12.53 years. Two of the 39 (5.1%) had B12 below the study's cut-off. One had Crohn's disease and had needed a further resection for ileal obstruction, with ongoing small bowel disease. The other had ulcerative colitis, and no cause other than the ileostomy was found. There was no relationship between B12 levels and how long someone had lived with the stoma, and no significant difference between the Crohn's and colitis groups (Jayaprakash 2004).

The authors concluded that routine B12 screening was not needed in this group unless someone had undergone additional small bowel resection or had ongoing small bowel inflammation. That is a small study, and 39 people cannot rule out a modest risk. But it is the study most directly about the question people type into a search bar, and it points the same way as the wider IBD literature: a systematic review of 42 studies covering 3,732 patients found that ulcerative colitis did not predispose to deficiency, and that Crohn's disease without ileal resection, wherever it sat in the ileum, did not increase the risk either (Battat 2014).

So the useful question is not "do I have an ileostomy?" but "what exactly was taken out, and is any of what remains inflamed?" The 2023 ESPEN nutrition guideline frames its B12 advice the same way, around resected and diseased ileum rather than around the stoma (Bischoff 2023).

Which part of the bowel absorbs vitamin B12?

The terminal ileum, the final part of the small intestine before it joins the colon. B12 from food binds a carrier protein made in the stomach, travels through the small bowel, and is taken up by receptors in the distal ileum. The colon plays no part, which is why removing colon alone does not affect B12 absorption.

This chain has several links, and a break in any of them causes deficiency. The stomach has to make acid and the carrier protein (intrinsic factor); the pancreas has to release B12 from the proteins it arrives on; and the ileum has to have enough healthy lining with the right receptors to take the complex in. Autoimmune gastritis breaks the first link, which is why it is the most common cause of deficiency in the general population. Surgery on the ileum breaks the last one.

One supplier page in the search results says absorption happens only in the last 3 to 4 cm of the ileum. The surgical data do not fit that. In a Canadian series of 33 patients who had 36 ileal resections including the terminal ileum, Schilling test results (an older radioactive measure of B12 absorption) were proportional to the length of ileum removed up to 60 cm (Thompson 1977). If only a few centimetres did the work, losing 20 cm and losing 50 cm would look the same. They did not.

Two other claims in circulation are worth correcting. A Reddit poster who had just had their colon removed asked how they would now absorb B12, A, D and K, believing these were absorbed in the large intestine (r/ostomy thread). B12 is not absorbed in the colon at all. And in another thread, a commenter said they had been told B12 is made by "a gland" in the intestine. It is not made by the body; it comes from food or supplements.

How much of the ileum has to be removed before B12 becomes a problem?

There is no clean cut-off, which is why your own operation note matters. Removing 60 cm or more of ileum almost always impaired absorption in older studies. A systematic review linked resections over 30 cm to deficiency, found under 20 cm was not, and called 20 to 30 cm inconsistent. Some studies found problems even after short resections.

This is the question patients ask most directly. One thread on r/ostomy is titled simply "Anybody lose the lower 18 inches of small bowel?", which is about 46 cm, and the replies range from people with no problems after similar lengths to people who lost far more (r/ostomy thread). The studies are not much tidier. Here is what each found:

The two Danish and Canadian series in the middle of the table disagree directly about short resections. Duerksen found no abnormal tests in 14 people with under 20 cm removed; Behrend found reduced absorption in 38% of people with 10 cm or less. We cannot reconcile them from the abstracts. Plausible contributors are that Behrend's patients all had an ileorectal anastomosis (their colon was gone, which changes transit), and that Behrend himself noted the absorption test flipped between normal and abnormal over time in several patients. Duerksen also only included people whose remaining terminal ileum was documented as normal, which removes active inflammation as a cause.

Ward's study adds a twist. It found that ileal inflammation, not just resection, raised the odds of deficiency (odds ratio 3.9), and that even short resections mattered. That contradicts the review's conclusion that Crohn's without resection does not raise risk. The difference is probably the test: Ward used holotranscobalamin backed by methylmalonic acid, which picks up more people than serum B12 alone, as the testing section below explains.

What this means for you. If your operation removed 60 cm or more of terminal ileum, the evidence says absorption is almost certainly reduced and the question is not whether to treat but how, which is your team's call. If it removed under 20 cm and the rest of your ileum is healthy, most studies suggest low risk, though not zero. Between those, nobody can predict it from length alone. That is why checking beats guessing.

Ileostomy for ulcerative colitis vs Crohn's disease: why the risk differs

People with an ileostomy for ulcerative colitis usually keep their terminal ileum, and studies find most absorb B12 normally. Crohn's disease is different: it often affects the terminal ileum itself and often needs ileal resection, so people with Crohn's and a stoma are the group where reduced absorption keeps turning up.

The Danish whole-body counting study shows the split clearly. Among eight people with ulcerative colitis and a conventional ileostomy, absorption was normal in most, with a median of 54%; one had markedly reduced absorption and two were borderline. Among seven people with Crohn's and ileal resection, all but one had reduced absorption (Bayat 1994). The groups are tiny, but the direction matches everything else.

In a US tertiary centre review of 201 people with Crohn's, an abnormal serum B12 was found in 18.4%, against 5% of 40 people with ulcerative colitis. A previous ileal resection raised the odds about sevenfold (odds ratio 7.22) and an ileocolonic resection about sixfold (5.81). Disease location and duration were not independently linked once surgery was accounted for (Headstrom 2008). The authors recommended routine screening in Crohn's, which is a stronger position than the end-ileostomy study took for its mixed group.

Not every study sees much of a gap. A prospective pilot of 66 people with Crohn's and 30 with ulcerative colitis found serum B12 below range in 7.6% and 10% respectively, and true deficiency (confirmed by methylmalonic acid) in 3% and 3.3%. Even people with more than 30 cm of ileum removed, whose average B12 was lower, had no confirmed deficiency in that sample (Battat 2017). Small numbers again, and a strict definition.

Why care, if the numbers are modest? Because when deficiency does happen, it seems to matter to how people feel. In a German survey of 783 people with a permanent end ileostomy, 44% for ulcerative colitis and 38% for Crohn's, B12, iron and zinc deficiencies were associated with lower quality of life, and B12 deficiency in particular with lower mental and emotional quality of life (Schiergens 2017). That is an association in a cross-sectional survey, not proof that B12 caused it. Our post on life after colectomy for ulcerative colitis covers the wider quality-of-life picture from that survey.

A 2025 scoping review that searched five databases for chronic nutrition-related problems after an ileostomy found only five studies on anaemia or B12, with deficiency rates "from no association to 31.8%" in its abstract. Its own results section gives the range as 5.1% to 31.2%, so one of the two figures is a typo; we could not tell which. Its authors blamed the spread on inconsistent definitions, different stoma types and reasons for surgery, and not knowing who was deficient before the operation (Leow 2025). That honest summary is the state of the field.

Can a J-pouch or a Kock pouch cause B12 deficiency?

Sometimes, and the studies disagree on how often. One series of 171 J-pouch patients found low B12 in 25%, with levels steadily falling in most people tested repeatedly. Another found deficiency in about 5% and tied poor absorption to inflammation in the ileum above the pouch. Both support checking during long-term follow-up.

A pouch is built from the end of the ileum, the same stretch that absorbs B12, and it can develop inflammation (pouchitis) or bacterial overgrowth, either of which could in theory interfere. The data are split:

• Coull 2007, Scotland. 171 patients with a restorative proctocolectomy, median follow-up 5.4 years. Abnormally low B12 in 25%. Of those with three or more measurements, 66% showed steadily declining levels. Yet 94% of those with low B12 had a normal absorption test and no bacterial overgrowth on breath testing, so the mechanism was unclear (Coull 2007).
• Kuisma 2001, Finland. 104 J-pouch patients at least five years out, plus 21 with a conventional ileostomy. B12 deficiency in about 5% of pouch patients. The lowest B12 and bile acid absorption was in people with inflammation in the ileum above the pouch (Kuisma 2001).
• Bayat 1994, Denmark. All 10 people with an ileal pouch had normal absorption on whole-body counting (Bayat 1994).

The Coull finding drew a published letter whose title asks whether pouch-related B12 deficiency is "really related to surgical procedure" (Sikorski 2008). We could not read the letter's text, so we cite it only to show the finding was contested. Coull's own absorption tests point the same way: if 94% absorbed B12 normally, something other than the pouch's absorbing capacity was lowering levels in many of them.

The older continent ileostomy (Kock pouch), an internal reservoir emptied with a catheter, has the longest follow-up. In 213 assessable patients followed 3 to 13 years, 7% developed subnormal B12 and another 7% were borderline. Small bowel resection had been added in 11 of the 14 with subnormal values, and subnormal or borderline values were more common in Crohn's (27%) than ulcerative colitis (12%). No one had anaemia or neurological symptoms from B12 deficiency, and the authors advised regular follow-up rather than routine supplements for everyone (Nilsson 1984). An earlier study of 34 Kock pouch patients found absorption normal or borderline in almost all, and showed the reservoir lining itself could absorb B12 (Jagenburg 1975).

For a pouch, then, inflammation in and above the pouch looks like the thing to watch. Our guides on what to expect after J-pouch surgery and how to track symptoms with a J-pouch cover the pouchitis side; adding your B12 results to that log costs one line.

How long after surgery does B12 deficiency show up?

Usually years, not weeks or months. The body holds a store of B12, so a drop in absorption takes time to show. In the longest ostomy follow-up we found, low B12 appeared a median of 7.5 years after surgery, ranging from 3 to 11 years. Feeling unwell a few weeks after surgery is very unlikely to be B12.

The pages that rank for this question each give a different store-depletion figure: one says monitoring should begin around two years after surgery, another says two to three years, a third says two to four. None of them gives a source. The one measured figure we found in an ostomy population comes from the Kock pouch study above: a median of 7.5 years from surgery to subnormal values (Nilsson 1984). That is one study of one type of operation, but it is a measurement rather than a rule of thumb, and it argues for checks spread over the long term, not a single test at year two.

Two practical consequences follow:

1. A normal result a year after surgery does not close the question if your ileum was resected. Coull found B12 drifting downwards over sequential tests in most pouch patients measured three or more times (Coull 2007).
2. Symptoms in the first weeks point elsewhere. One r/ostomy poster described weeks of very watery output followed by muscle pain, constant tingling, a pounding heart and poor sleep, and assumed B12 deficiency despite taking B12 tablets several times a day (r/ostomy thread). Several of those symptoms overlap with dehydration and salt loss, which a high-output ileostomy can cause within days. The ESPEN guideline notes that a high-output stoma can lead to dehydration, malnutrition and acute kidney injury (Bischoff 2023).

If your output has been high and you feel unwell, contact your stoma nurse or doctor the same day rather than waiting for a B12 result. The free checker below places your 24-hour output against published figures and flags dehydration warning signs.

What are the symptoms of B12 deficiency with an ileostomy?

Tiredness, brain fog, pins and needles or numbness, balance problems, a sore smooth tongue, eyesight changes and anaemia are the commonly listed signs. None is specific, and several overlap with dehydration and iron deficiency, both common with an ileostomy. Anaemia does not have to be present: nerve symptoms can come first.

The 2024 NICE guideline lists these as common symptoms and signs: abnormal blood count results such as anaemia or large red cells; cognitive difficulties such as poor concentration or short-term memory loss, sometimes described as brain fog; eyesight problems including blurred vision; glossitis (a sore, smooth, red tongue); neurological or mobility problems including balance problems, falls and an impaired gait; pins and needles or numbness; and unexplained fatigue. It also says not to rule out deficiency just because there is no anaemia or enlarged red cells, and that deficiency can be associated with depression, anxiety or psychosis (NICE NG239).

The overlap problem is real for people with a stoma. In the r/ostomy thread that ranks for this search, the original poster, five years after an ileostomy for colitis, described waves of brain fog, light-headedness, extreme fatigue, trembling legs and tingling hands. Their blood count was normal and only vitamin D came back low. Replies split between "get your B12 checked specifically, a blood count will not show it" and "this sounds more like dehydration" (r/ostomy thread). Both replies have a point. A full blood count does not measure B12, so you need to ask for it. And dehydration can produce almost the same list.

Fatigue in IBD has many causes, and our posts on Crohn's fatigue in remission and ulcerative colitis fatigue and low iron work through them. The short version for this question: B12 is worth testing when you are at risk, but it explains fatigue in a minority of people.

Which blood test checks B12, and can a normal result be wrong?

The first test is usually total serum B12 or "active B12" (holotranscobalamin). Results in a middle band are indeterminate, and a follow-up test, methylmalonic acid (MMA), helps decide. A normal serum B12 can miss deficiency, and a low one can overcall it, so the result should be read alongside symptoms and your surgical history.

NICE sets these thresholds, while allowing labs to use their own where local values differ substantially (NICE NG239):

For an indeterminate result, NICE suggests considering an MMA test in people with symptoms, and considering replacement while waiting in people who have had surgery that can cause deficiency, naming terminal ileal resection (NICE NG239). The British haematology guideline is blunter about the underlying problem: there is no gold standard test, cut-offs cannot be defined precisely, and if strong clinical features clash with a normal-looking result, treatment should not be delayed because nerve damage is the risk (Devalia 2014).

How much the test choice matters is shown by two IBD studies that reach almost opposite conclusions:

• In a UK Crohn's cohort, 89 people had both tests. Standard serum B12 identified deficiency in 5%; holotranscobalamin backed by MMA identified it in 32% (Ward 2015).
• In a Canadian pilot, serum B12 was below range in 7.6% of people with Crohn's, but when deficiency was defined as low B12 plus raised MMA, only 3% were truly deficient (Battat 2017).

One study says serum B12 misses most deficiency; the other says it overcalls it. They used different first-line tests and different definitions, and we cannot tell you which is right. What both agree on is that a single serum B12 is not the whole story. The ESPEN intestinal failure guideline reaches the same view, calling a combination of holotranscobalamin and MMA optimal, with serum B12 as a fallback (Pironi 2023).

How often should B12 be checked after an ileostomy?

It depends on your group. ESPEN advises yearly screening for people with Crohn's who have ileal involvement or resection. An end-ileostomy study advised against routine screening unless there was extra small bowel resection or ongoing small bowel inflammation. Pouch studies support checks during follow-up. Your surgical history decides which applies.

Putting the guidance side by side:

Two gaps are worth knowing about. First, NICE's list of common risk factors names terminal ileal resection, gastrectomy, bariatric surgery, certain medicines, diet and autoimmune conditions, but on the recommendations page we read it does not mention Crohn's disease, inflammatory bowel disease or ileostomy at all (NICE NG239). If your operation is described as "colectomy and ileostomy" rather than "ileal resection", a busy clinician working from that list might not flag you. Second, we found no guideline that addresses B12 after an end ileostomy for ulcerative colitis directly; ESPEN's B12 recommendations are written for Crohn's (Bischoff 2023).

If your B12 is already checked as part of routine monitoring on a biologic or immunosuppressant, ask whether it is on the panel; our explainer on why you need blood tests on a biologic covers what those panels usually include.

Will B12 tablets work with an ileostomy, or do you need injections?

That is a decision for your doctor, and the evidence is thinner than either side suggests. NICE advises lifelong injections after complete terminal ileal resection. A study of people with Crohn's found oral B12 normalised levels in most, but the randomised trials comparing tablets and injections largely excluded people with IBD.

This section reports evidence. It is not a recommendation to start, stop or switch anything.

• NICE (2024) recommends offering lifelong intramuscular B12 to people who have had a complete terminal ileal resection. For malabsorption from other causes, it suggests considering injections over tablets (NICE NG239).
• ESPEN (2023) says parenteral (injected) B12 remains the reference in Crohn's, that oral therapy may be as effective but is poorly explored, and that oral may become standard in coming years. Its recommendation that B12 be given after more than 20 cm of distal ileal resection is graded B with 100% consensus (Bischoff 2023).
• A Spanish multicentre study of 94 people with Crohn's and B12 deficiency found oral B12 normalised levels in 94.7% of 76 treated for deficiency, and kept levels normal in 81.7% of 82 on maintenance over a mean of three years. Only 21 had an ileal resection, and two of the four who did not normalise were in that group. Among maintenance failures, 46.6% admitted not taking the tablets regularly. It was retrospective and not randomised; the authors declared no conflicts (Gomollon 2017).
• A Cochrane review found three randomised trials with 153 participants comparing oral and injected B12. Low-quality evidence suggested similar effects on blood levels, and none of the trials measured symptoms (Wang 2018). The Spanish authors point out that two of those trials excluded people with IBD and the third did not report how many had Crohn's (Gomollon 2017).

The mechanism behind oral treatment is passive absorption: a small fraction of a large dose crosses the gut lining without intrinsic factor or the ileal receptors. The Spanish paper cites a 1968 study reporting 0.5% to 4% absorbed this way along the whole intestine (Gomollon 2017); we could not read that original paper. Whether passive absorption is enough when most of the ileum is gone and transit is fast is exactly the question the trials did not answer.

Supplier and charity pages in the search results say flatly that oral B12 is not a reliable route after an ileostomy. That is an overstatement in one direction; "tablets work fine for everyone" would be an overstatement in the other. For someone with a complete terminal ileal resection, the guideline position is injections. For everyone else, the route is a conversation with your team, with your adherence and preferences part of it.

One more trial matters here because Reddit threads often recommend B12 for tiredness. In a double-blind trial of 95 people with IBS or IBD, fatigue and normal B12 levels, eight weeks of high-dose oral B12 reduced fatigue by 8.1 points against 8.3 on placebo: no difference (Scholten 2018). Treating a confirmed deficiency is one thing; taking B12 for fatigue when levels are normal did not help.

What else stops being absorbed properly after an ileostomy?

Water and sodium are the main losses with any ileostomy, because the colon normally reclaims them. If a long stretch of terminal ileum was also removed, bile acids and fat can be poorly absorbed. Folate deficiency in IBD is linked to the upper small bowel and to medicines rather than to the ileum.

• Water and salt. This is the everyday absorption issue with an ileostomy and the one most likely to make you feel unwell quickly. ESPEN recommends monitoring output and urine sodium in people with a high-output ileostomy (Bischoff 2023). Our J-pouch vs ostomy comparison and how to track food triggers with an ostomy cover hydration and output in detail.
• Bile acids and fat. The terminal ileum also reabsorbs bile acids. In the Canadian series, seven people with 60 cm or more of ileum removed had stool-fat tests and none were normal, while eight of ten with less removed were normal (Thompson 1977). The Finnish pouch study found the lowest bile acid absorption alongside inflammation in the ileum above the pouch (Kuisma 2001). If you still have a colon after ileal resection, bile acids reaching it cause diarrhoea; our bile acid malabsorption post covers that.
• Folate. The ESPEN intestinal failure guideline attributes folate deficiency in IBD to problems higher in the small bowel and to medications, not to the ileum (Pironi 2023).
• Fat-soluble vitamins (A, D, E, K). Low vitamin D comes up constantly in ostomy threads. In the Finnish pouch study, vitamin D deficiency affected 10.6% of pouch patients, and vitamin E was significantly lower in those whose pouch lining showed severe villous atrophy (Kuisma 2001).

Can you get B12 deficiency with a colostomy or after colon resection?

Yes, but not because of the colostomy. The colon plays no part in absorbing B12. Low B12 with a colostomy usually has the same causes as in anyone else: diet, autoimmune gastritis, certain medicines such as metformin and acid-reducing drugs, or age. If an operation removed colon plus some ileum, that ileal part is what matters.

A poster on r/ostomy who had about a foot of colon removed asked exactly this, having heard B12 deficiency was common with ileostomies (r/ostomy thread). The replies were mixed, but one made the key point: B12 absorption does not involve the colon, and a blood test comes first. Another described developing deficiency with an almost intact colon, probably for a different reason.

NICE's list of common risk factors is a good checklist for those other reasons: a diet low in animal foods without fortified foods or supplements; a family history of B12 deficiency or autoimmune conditions; autoimmune gastritis, coeliac disease or thyroid disease; medicines including metformin, proton pump inhibitors, H2 blockers, colchicine and some anticonvulsants; previous abdominal or pelvic radiotherapy; and gastric or bariatric surgery (NICE NG239). If the operation that gave you a colostomy was a right-sided one (a right hemicolectomy or ileocaecal resection removes the end of the ileum along with the start of the colon), the ileal resection questions above apply.

A worked example: three people, three different answers

These are composites built to show how the evidence applies, not real patients.

Person A, 52, end ileostomy for ulcerative colitis eight years ago. Colectomy only, no further surgery, no small bowel disease. She is tired and wonders about B12. On the evidence, her absorption is probably normal (Bayat 1994), and the end-ileostomy study would not have screened her routinely (Jayaprakash 2004). But she has a symptom, so testing is reasonable, and so is checking hydration, iron and thyroid at the same time, because those are likelier explanations. If B12 comes back indeterminate, the MMA step decides.

Person B, 34, Crohn's, ileocaecal resection of 45 cm, later an ileostomy. He feels fine. On ESPEN's guidance he falls into the group where B12 should be given rather than just monitored, because more than 20 cm of distal ileum was removed (Bischoff 2023), and about half of people with resections in his range malabsorb B12 on absorption testing (Duerksen 2006). Feeling fine does not settle it, because stores take years to run down. His job is to make sure someone has decided, in writing, what his B12 plan is.

Person C, 41, J-pouch for ulcerative colitis twelve years ago, two episodes of pouchitis. Her B12 has been "normal" three times, each lower than the last. Coull's series found exactly this downward drift in most pouch patients tested repeatedly (Coull 2007), and the Finnish study tied low absorption to inflammation above the pouch (Kuisma 2001). A trend inside the normal range is worth raising, and so is any recurring inflammation.

Same word, "ileostomy" or "pouch", three different risk levels. What separates them is the operation note and the disease history, not the stoma.

What to write down so your B12 risk can be judged in a minute

Most of the uncertainty above disappears if whoever is looking at your results knows exactly what surgery you had. Keep a short record:

1. Every bowel operation, with the date and its name as written on the discharge letter (for example "subtotal colectomy and end ileostomy", "ileocaecal resection", "completion proctectomy").
2. How much small bowel was removed, in centimetres. It is usually in the operation note or the pathology report. If you do not know, ask the surgical team to look it up; it is the single most useful number in this whole article.
3. Whether any remaining small bowel is affected by Crohn's, and the date of the last scan or scope that checked.
4. Every B12 result, with the date, the test (total or active B12, MMA if done) and the lab's reference range, so a slow drift is visible.
5. Any B12 you take that was not prescribed, including multivitamins, sprays and patches, and when you took them relative to a blood test.
6. Symptoms that might fit: numbness, tingling, balance problems, memory or concentration changes, and when they started.

If you keep this in an app, Clairop lets you log symptoms by voice or text and produces a one-page GI visit report covering bowel pattern, medication adherence and lab results that you can hand over at an appointment.

Myths worth dropping

"Everyone with an ileostomy will become B12 deficient." The studies that measured it in end-ileostomy patients found deficiency in a small minority, concentrated in people who had also lost ileum or had ongoing small bowel Crohn's (Jayaprakash 2004; Nilsson 1984).

"B12 is absorbed in the large intestine." It is absorbed in the terminal ileum. Removing the colon alone does not change it.

"It shows up two years after surgery." That figure circulates without a source. The one measured median we found was 7.5 years, with a range of 3 to 11 (Nilsson 1984).

"A full blood count would have shown it." A blood count can show anaemia or large red cells, but it does not measure B12, and NICE says deficiency should not be ruled out just because there is no anaemia (NICE NG239).

"A normal B12 rules it out." In one Crohn's cohort, serum B12 found deficiency in 5% while a more specific combination found 32% (Ward 2015). An indeterminate result in someone with ileal resection is a reason for a follow-up test, not reassurance.

"B12 shots will fix the fatigue." Not if your B12 is normal: high-dose B12 did no better than placebo for fatigue in IBD and IBS (Scholten 2018).

"Short resections are always safe." Usually, but one study found reduced absorption in 38% of people with 10 cm or less removed (Behrend 1995), and another linked even resections of 20 cm or less to deficiency (Ward 2015).

When to see a doctor promptly

The honest bottom line

Having an ileostomy is not, by itself, a strong reason to expect B12 deficiency. Losing terminal ileum is. If you had a colectomy for ulcerative colitis and kept your ileum, studies put your risk low, and the one study that looked did not recommend routine screening, although testing when symptoms appear is sensible. If you have Crohn's and lost a meaningful stretch of ileum, current guidance says B12 should be given or at least checked yearly, and feeling well is not reassurance because stores fade over years. If you have a pouch, the studies disagree on how often B12 falls, which is itself a reason to check.

The questions nobody can answer cleanly yet are the ones that matter most to individuals: exactly which resection length is safe, which test to believe when they disagree, and whether tablets are enough after major ileal loss. What you can control is the record. Know how many centimetres were removed, keep your results with dates, and make sure someone on your team has written down a plan.