Vitamin B12 (cobalamin) in cats: low or high, it's a number that asks why
Cobalamin (vitamin B12) shows up on more and more blood test reports. A low result usually ends with "get the injections" and a high one with "that's fine", but a 2026 UK study paints a different picture. Measuring every cat that had blood drawn, 229 in all, it found one in three high and one in five low (Casas & Dye 2026).
This article sets out, with the numbers from the cat studies published so far (plus a few in dogs), what low and high B12 each mean and how supplementation works.
- Low B12 signals an absorption problem. It is common with gut disease (inflammatory bowel disease, lymphoma), exocrine pancreatic insufficiency (77% low), old age (the median almost halves after 13 years) and body weight under 3 kg. About 20% of cats without gut signs were low too.
- Supplementing is easy and safe. Injections (250 µg weekly for 6 weeks) or tablets (250 µg daily): in the only randomised trial in cats, both normalised 100% (Chang 2024, 48 cats). No toxic upper limit is known.
- It drops again when you stop. Cats that finished 6 weeks of injections were back near baseline 10 weeks later (Kempf 2017). Treat the underlying disease and recheck.
- High B12 without supplementation is "meaning unknown". One study linked it to liver disease and tumours (Trehy 2014), another found no link (Henry 2023), and one found longer survival in the high group (Borgonovi 2025). No need to panic, but if other abnormalities overlap it is a clue to check the liver and gut.
- A normal value does not rule out deficiency. 23% of cats with cellular deficiency (raised MMA) had B12 above 900 pg/mL (Worhunsky 2013).
| Who | Decides |
|---|---|
| You, at home | Recording weight, stools and appetite; giving oral B12 every day without gaps; keeping the recheck date |
| Together | When to test (old age, weight loss, gut signs), injections or tablets, recheck interval |
| Your vet | Finding the cause of low B12 (endoscopy, ultrasound, pancreatic tests); whether high B12 needs liver or tumour work-up |
1. What B12 does, and how cats absorb it
B12 is a coenzyme used to make DNA, metabolise fats and amino acids, sheath nerves and mature red blood cells. Deficiency commonly brings weight loss, poor appetite, diarrhoea and vomiting, and rarely neurological signs (hind-limb ataxia) have been reported.
Cats absorb it in a way of their own.
- They make intrinsic factor (the protein that carries B12 into the body) only in the pancreas. People also make it in the stomach; cats do not. So exocrine pancreatic insufficiency almost always lowers B12.
- Absorption happens at the end of the small intestine (ileum). Inflammation or lymphoma there blocks it.
- Stores run down quickly. The half-life of injected B12 was 12.75 days in 4 healthy cats and 5 days in 2 cats with enteritis (Simpson 2001). The numbers are tiny, but it means stored B12 also leaves faster when the gut is unwell.
2. Reading the number
| Range (pg/mL = ng/L) | Meaning | Usual action |
|---|---|---|
| Under 250 | Low | Supplement + find the cause |
| 250–399 | Borderline: cellular deficiency may already be present | The Texas A&M GI Lab recommends supplementing from this range |
| 400–1000 | Normal (upper limit varies by lab) | — |
| Over 1000 | High | Expected if supplementing; otherwise interpret with other findings |
Ranges are those of the Casas 2026 study (Immulite analyser). For reports in pmol/L, multiply by 1.355 to get pg/mL (e.g. 300 pmol/L ≈ 407 pg/mL).
- Cut-offs differ by lab and analyser. A 2005 UK study counted "over 150" as normal, so almost no cats were low, while other studies used 305–900 as the lower limit. Compare values from the same lab.
- The upper limit is uncertain too. In 65 healthy cats on the same diet the range was 261.7–1169.7 pmol/L (about 355–1585 pg/mL) (Hill 2018). A value just over 1000 may be flagged "high" and still be normal.
- Kittens run low. 13 of 14 healthy kittens were below the adult range at 3 months, and 4 were still low at 12 months (Stavroulaki 2022).
Inside cells, B12 does two jobs. One is helping the enzyme that moves 'methylmalonyl', an intermediate of energy metabolism, on to the next step; the other is converting homocysteine to methionine. When cells run short of B12 the first job stalls, methylmalonyl builds up and leaves in blood and urine as MMA.
So the two mean different things. Blood B12 is "stock in transit"; MMA shows "whether the factory floor is actually short". Normal blood B12 with high MMA means the cells are deficient.
- How far apart they can be: of 48 cats with deficiency confirmed by MMA, 23% had blood B12 above 900 pg/mL and 45% above 290 (Worhunsky 2013). There is also a cat case with very high blood B12 alongside high MMA and ammonia (Choi 2023).
- Measuring during supplementation widens the gap: at week 6 of injections blood B12 was 2,332 pmol/L, far above range, yet 12 of 20 cats still had abnormal MMA (Kempf 2017). Blood levels rise as soon as you give it, but cells fill up later. A high value measured during supplementation means "high because it's being given", not that the body is replete; that is why the recheck comes a month after finishing.
- Why they diverge: in people, much of blood B12 is bound to a carrier protein (haptocorrin) that does not deliver it to cells, so the total can mislead. Cats' carrier proteins differ from ours, so whether this applies directly is not yet known. Inflammation and liver disease can also raise blood levels alone (section 6).
- Limits: MMA is not a routine blood test and often has to go to a specialist lab, and kidney failure itself raises MMA, making it hard to read in cats with poor kidneys. At or below 209 pg/mL, deficiency is likely even without MMA (specificity 96%).
3. Every study so far, in one table
Click a study name to open its abstract (PubMed).
| Study | Subjects · design | Result | What to note |
|---|---|---|---|
| 🐱 How common | |||
| Casas & Dye 2026 | 229 UK referral cats, every cat with blood drawn (prospective) | High 34.9% · low 19.7% · borderline 7.9%. Lower with age and under 3 kg. Gut disease 18.6% vs other disease 20.3% | Less biased than studies of cats tested because B12 was suspected. Referral population |
| Borgonovi 2025 | 216 UK referral cats (retrospective) | Low 35% · high 31%. Median survival low 274 days vs high 711 days | Low carries the worse outlook. Chronic enteropathy was the main diagnosis in both groups |
| Hill 2018 | 65 healthy cats on the same diet | B12 falls with age. Normal range about 355–1585 pg/mL | The diet held 17 times the AAFCO minimum, so diet can shift values |
| 🐱 When low: underlying disease | |||
| Simpson 2001 | 80 cats with suspected gut disease | 49 (61%) low, lymphoma especially | Used a high lower limit of 900 pg/mL |
| Kook 2012 | 261 cats with gut disease | 41.4% low (26.4% below the detection limit). Vomiting plus diarrhoea: 2.9 times the odds | Cats below detection also had lower haematocrit and albumin |
| Jugan 2017 | 75 cats with low B12 (retrospective) | Only 65% had small-intestine changes on ultrasound | B12 can be low with a normal-looking scan |
| Xenoulis 2016 | 150 cats with exocrine pancreatic insufficiency | 77% low. Cats given B12 responded to treatment 3 times better | Questionnaire-based. Authors from the lab that sells the test |
| Cook 2011 | 76 hyperthyroid vs 100 euthyroid older cats | Low 40.8% vs 25% | The two studies disagree. B12 may normalise after thyroid treatment, so without gut signs there is little rush |
| Geesaman 2016 | 39 hyperthyroid cats before and after radioiodine (prospective) | Only 13% low, no raised MMA, 2 normalised after treatment | |
| Guse 2025 | 144 diabetic cats | 24.5% low | Laboratory company data |
| Kiselow 2008 | 41 cats with low-grade lymphoma | 78% of those tested were low. Median survival 704 days; B12 not related to survival | "Low B12 means a worse lymphoma outlook" is not this study's finding |
| 🐱 When high | |||
| Trehy 2014 | 42 of 174 cats high without supplementation | Odds ratio: liver disease 9.9, solid tumours 8.5, pedigree 4.2 | Very wide confidence intervals (tumours 1.1–66) |
| Kather 2020 | 34 of 323 cats high (11%) | Chronic enteropathy 65%, pancreatitis 24%, cholangiohepatopathy 18% | High B12 is also common with gut disease |
| Henry 2023 | 8 high cats vs controls | No link with cancer | Very small numbers |
| 🐱 Supplementation | |||
| Chang 2024 | 48 cats under 290, randomised: oral 250 µg for 12 weeks vs SC for 10 weeks | B12 normalised 100% in both. MMA normalised 70% oral vs 82% SC (gut disease), 88% in both (pancreatic) | The only randomised trial in cats. All authors at the lab that sells the tests; some consult for a supplement company |
| Toresson 2017 | Oral cyanocobalamin 0.25 mg daily, 25 cats (retrospective) | All 25 rose above the reference interval (median 128 → 2701 pmol/L) | Tablets raise it well |
| Kempf 2017 | IM 250 µg weekly for 6 weeks, 20 cats | Week 6: 2332 → week 10: 610 → week 16: 180 pmol/L (baseline). MMA not normal in 12 cats at week 6 | It drops after stopping and signs return |
| Kook 2020 | Hydroxocobalamin 300 µg every 2 weeks ×4, 23 cats | MMA normalised 22/23, clinical score 6 → 1 | A longer-lasting form, so injections can be spaced out |
| Ruaux 2005 | 19 cats at or below 100, 250 µg weekly for 4 weeks | MMA 5373 → 424, weight +8.2%, vomiting improved 7/9, diarrhoea 5/13 | Evidence that supplementing improves real signs |
| 🐶 Dogs | |||
| Toresson 2018 | 53 dogs, randomised: oral vs injections | All within or above range at 28 days. Day 90 median oral 1244 vs injected 877 ng/L | Tablets work as well as injections in dogs too |
| Allenspach 2007 | 70 dogs with chronic enteropathy | B12 under 200 a risk factor for treatment failure (euthanasia) | Low B12 is a prognostic signal in dogs too |
| Batchelor 2007 | 178 dogs with exocrine pancreatic insufficiency | B12 under 100 linked to shorter survival | — |
| Berghoff 2013 | 56 dogs with gut disease | 36% low, only 25% of them with raised MMA | In dogs, low often isn't cellular deficiency, unlike cats |
In short: low B12 reflects a problem in the absorption route (gut, pancreas) and rises reliably with supplementation, while the meaning of high B12 is not yet settled. That is where things stand.
4. When it's low: look for the cause first
B12 is less a disease in itself than a sign that the gut or pancreas is not working properly. Diets short of B12 are rarely the reason (section 8).
- Chronic enteropathy and lymphoma: the commonest causes. 28–61% of cats with gut disease were low (varies by study and cut-off). B12 can be low with a normal ultrasound.
- Exocrine pancreatic insufficiency: 77% low. Suspect it when a cat loses weight while eating a lot and passes greasy stools.
- Old age and low body weight: in the 2026 study the median was 440 pg/mL in cats over 13 (1000 at 3–8 years) and 443 under 3 kg. Values tend to fall even without a specific disease.
- Hyperthyroidism and diabetes: low results are more frequent, but in hyperthyroidism B12 sometimes normalises after treatment and often isn't a cellular deficiency (Geesaman 2016).
5. How to supplement: injections vs tablets
| SC or IM injection | Tablets | |
|---|---|---|
| Dose (cats) | 250 µg weekly × 6 weeks → 1 dose a month later → recheck a month after that | 250 µg once daily × 8–12 weeks → recheck 4 weeks after stopping |
| Evidence | Chang 2024: normalised 100%, MMA normalised 82% (gut disease) | Chang 2024: normalised 100%, MMA normalised 70% (gut disease) |
| Pros | Definitely absorbed; no daily dosing | Fewer clinic visits; no injection stress |
| Cons | Weekly clinic visit or home injection | Must be given every day without fail |
Doses follow the Texas A&M GI Lab protocol. Intramuscular hydroxocobalamin 300 µg every 2 weeks ×4 has also been shown to work (Kook 2020).
- Don't stop without a recheck. If the cause remains, it falls again (Kempf 2017: back to baseline 10 weeks after the last injection).
- What you gain: weight +8.2%, less vomiting and diarrhoea (Ruaux 2005), clinical score 6 → 1 (Kook 2020). Many owners report appetite returning after supplementation, but no controlled study has measured appetite on its own.
- The normal route is capacity-limited. B12 binds intrinsic factor and enters through receptors at the end of the small intestine, and that route can only take so much at a time. Gut disease or exocrine pancreatic insufficiency blocks it.
- At high doses a side door opens. Some B12 crosses the gut wall by concentration difference alone, without intrinsic factor. In people it is widely cited as about 1% of the dose, starting from a study that treated pernicious anaemia patients, who lack intrinsic factor, with high oral doses (Berlin 1968). In a human randomised trial (38 people), 2 mg orally every day gave higher blood B12 than intramuscular injections at 4 months (1,005 vs 325 pg/mL) and lower MMA (Kuzminski 1998).
- Doing the maths for a cat: the requirement is 4.5–5.6 µg per 1000 kcal, so about 1.1–1.4 µg a day for a cat eating 200–250 kcal. 1% of 250 µg is 2.5 µg: even with the normal route blocked, about twice the daily need comes through the side door. That is why cat and dog B12 products come in 250 µg units for animals under 10 kg.
- What has been confirmed in cats: no study has measured what percentage diffuses in cats. But tablets normalised B12 even in exocrine pancreatic insufficiency, where intrinsic factor is nearly absent (Chang 2024 in cats, Chang 2022 in dogs). The 1% figure above is borrowed from humans.
Why a human B-complex is hard to use: to reach 250 µg of B12 you also give the other B vitamins (B1, B6, niacin and so on) at human doses. So cats end up getting 1/5 to 1/10 of a capsule, which cuts B12 to a few tens of µg, and what gets through the side door may fall short of the daily need. The strong B-vitamin smell also makes it hard to mix into food. If B12 is the goal, use a plain cyanocobalamin product (250 µg pet products, or a human single-ingredient product without xylitol or other vitamins). Agree the dose with your vet.
Cats on acid suppressants: B12 in food needs stomach acid to come off its protein. In people, 2 or more years of proton pump inhibitors raised the risk of B12 deficiency 1.65 times (Lam 2013, 26,000 deficient patients). Supplement B12 is not protein-bound to begin with, so it doesn't need acid. There are no cat data, but for cats on long-term omeprazole or famotidine, supplements are the more reliable route than food.
6. When it's high: check before you worry
- Is the cat being supplemented? Injections, tablets or a supplement containing B12 make high values normal (Toresson 2017: every cat above range on tablets).
- Is it a cut-off issue? Many labs set the upper limit at 1000, but the healthy-cat range reaches about 1585. Values of 1000–1500 may be flagged high and still be normal.
- Are there other abnormalities? If raised liver values, weight loss or gut signs overlap, use it as a clue for liver and gut tests. If B12 alone is high and everything else is fine, current evidence gives no specific next step.
Why it rises is not yet known. In people, the B12 carrier protein (transcobalamin) increases with inflammation, the kidneys clear it less, or damaged liver or tumour cells release stores. In cats, a small study found high transcobalamin and B12 in advanced kidney disease (Kunath 2024).
7. B12 in cats with kidney disease
Hardly any study has looked specifically at B12 in cats with CKD. Two things are known.
- A small study found B12 and transcobalamin high in cats with advanced CKD (Kunath 2024).
- Kidney failure raises MMA, so MMA is hard to use for judging cellular deficiency in cats with poor kidneys.
An earlier version of our drug and supplement compendium said "cats with CKD lose B12 in urine and many become low", but on rechecking we found no cat study showing this. For cats with CKD the rule is the same: test if weight is falling or there are gut signs, and supplement if low.
8. B12 in food: hard to run short
- The cat requirement is about 4.5–5.6 µg per 1000 kcal (NRC, AAFCO, FEDIAF), or 20 µg per kg of dry matter by AAFCO.
- Commercial foods usually contain far more. The diet in the Hill 2018 study had 350 µg per kg of dry matter, 17 times the minimum.
- So low B12 in a cat on commercial food is almost always an absorption problem. Finding the cause and supplementing beats changing food.
- For home-made diets, check that the recipe has enough B12 sources (liver, meat).
9. Where opinions differ
Investigate high B12 further? Links with tumours and liver disease vary between studies and the numbers are small. Our view: don't rush extra tests on B12 alone, but look together if liver values, weight or gut signs are also off.
How to check in your own cat: compare results from the same lab before supplementing and 1 month after finishing, and record weight and stools in between.
10. What to bring to the vet
- Weight change over the past 2–3 months: the app's chart as it is.
- Stool and vomiting: how many weeks it has been going on.
- Supplements being given: a multivitamin containing B12 explains a high value.
- Previous B12 results and the lab's name: if units (pg/mL, pmol/L) or cut-offs differ, line them up together.
11. Read next
- Inflammatory bowel disease
- Pancreatitis
- Anaemia in CKD: DPO and iron go together
- When other diseases come along
- Drug and supplement compendium: cobalamin
📌 One-line tip: when you get a B12 result, check the unit (pg/mL or pmol/L) and the lab's cut-offs first. The same number can be "low" or "normal" depending on the reference.