Vitamin B12

For a measured low level or a clear cause. Not for age alone: without a shortage it does nothing measurable.

At a glance

Our verdict	When a test shows low, or a cause exists (metformin, acid blockers, vegan diet). Not on age alone.
Best use	Low B12, or a cause for it
How sure the evidence is	Moderate that it does nothing without a shortage
Cost a month	About $3 to $5

Written for adults in Canada. It explains the evidence; it is not advice for you personally. Last reviewed 20 October 2026.

Supplement Check

Is it right for you?

Whether you need B12 depends on your diet, how long you have taken metformin or an acid blocker, any stomach or bowel surgery, any numbness or memory change, and your blood level. We work out whether a test should come first. Our Supplement Check looks at:

Whether there is a reason beyond age (age alone is not enough)
A measured low B12 (under 150 pmol/L)
Numbness or tingling in the feet or hands, unsteadiness, or new memory problems
Weight-loss surgery, or part of the stomach or bowel removed
A high B12 when you take none, or an unexplained high level
Every medicine you take
The total across everything you already take, against the safe upper limits

Coming soon. The Supplement Check and the Online Health Assessment open once our safety testing is complete.

$59 when it opens, paid once. Adults 18 and over, anywhere in Canada. While the service is new, Dr. French reads every report himself, and your report arrives within 3 working days. More about the Supplement Check

Tell me when it opens

What it is

Vitamin B12 (cobalamin) is found in animal foods and in fortified foods such as plant drinks, cereals and nutritional yeast. Supplements are usually cyanocobalamin (stable and cheap) or methylcobalamin.

We have not named a product yet; the label dose in our check is 1,000 micrograms a day. We do not sell B-complex products: most Canadian ones contain 50 mg or more of vitamin B6, and Dr. French's rule keeps B6 from all products under 10 mg a day.

Structure of Vitamin B12 — Structure: PubChem (public domain).

How it acts

B12 helps two enzymes. One turns homocysteine into methionine and recycles folate; the other handles certain fats. A shortage causes a large-cell anaemia and damage to the spinal cord and nerves. About 1% of a large dose by mouth is absorbed without the stomach's own carrier (intrinsic factor), which is why 1,000 micrograms by mouth works even when normal absorption has failed.

Outline of a person showing where Vitamin B12 acts in the body

What it is used for, and what the evidence says

B12 is used to treat and prevent a shortage: a vegan diet, pernicious anaemia, thinning of the stomach lining, metformin, long-term acid blockers, weight-loss surgery and bowel disease. Dr. French asked whether every adult over 50 should take it.

Use	What the trials found	How sure we are
Older people with a moderately low level, no anaemia	Age 75 and over, 1 mg a day for 12 months: no benefit for nerve function, the neurological examination, or memory and thinking, [1] even in those with the lowest starting levels. [2] One well-run trial.	Moderate
B12 without a shortage, for memory, mood or tiredness	A meta-analysis found no effect on memory and thinking, depressive symptoms or fatigue in people who were not deficient. [3]	Moderate
Metformin	In a large US trial, people on metformin were about twice as likely to have a low or borderline-low B12 at 5 years (19% against 10%), and the odds of deficiency rose about 13% for each year of use. [4]	Moderate
Acid blockers	Two years or more of acid-blocking pills went with about 65% higher odds of diagnosed deficiency in a US health-plan study. [5]	Low
Tablets against injections	In 3 small trials (153 people), 1,000 micrograms a day by mouth gave blood levels similar to injections; in one, 2,000 micrograms by mouth gave higher levels. [6]	Low
Who should be tested	The UK's National Institute for Health and Care Excellence (2024): test only when there are symptoms or a risk factor. [7]	Guideline
A high B12 level	A high level without supplements is a marker to investigate: in 333,667 Danes, levels over 800 pmol/L carried 6.3 times the expected cancer rate in the first year, which points to the illness raising B12, not B12 causing the illness. [8] Men who took single-vitamin B12 over 55 micrograms a day for 10 years had about twice the lung cancer risk, mainly in smokers (observational). [9]	Low

How sure we are: High means further research is very unlikely to change the answer; Moderate, it might; Low, it may well change; Very low, we are very unsure. These follow the GRADE method that guideline panels use.

Benefits

In a shortage: corrects the anaemia and stops the nerve damage progressing (established).
Tablets by mouth work as well as injections for most people, for a few dollars a month.
Without a shortage: no measurable benefit (moderate certainty).

Risks and side effects

Very few at normal doses. Acne-like rashes are rare.
A missed diagnosis. Taking B12 without a test can hide pernicious anaemia or a stomach problem.
Lung cancer in male smokers on long-term, high-dose single B12: a possible small effect (low certainty). [9, 10] Smokers get B12 only for a measured need.
Leber hereditary optic neuropathy: cyanocobalamin is avoided.

Interactions with medicines

B12 does not change how the liver clears medicines, and it has no clinically important interaction with them. Some medicines lower B12, which is why we ask about them.

Medicine	What can happen	What to do
Metformin	Lowers B12 over the years	B12 blood test
Acid blockers (omeprazole, pantoprazole, esomeprazole, lansoprazole, rabeprazole, famotidine) for 2 years or more	Lower B12 absorption	B12 blood test
Colchicine, nitrous oxide	Lower B12 status	Noted in your review

Who should not take it

Not on age alone: over 50 with no cause, we give a food line, not a tablet.
A measured low B12 (under 150 pmol/L): held for Dr. French to find the cause and set the dose.
Numbness or tingling in the feet or hands, unsteadiness, or new memory problems: book a consultation.
Weight-loss surgery, or part of the stomach or bowel removed: a B12 test, and Dr. French reviews.
A high B12 when you take none, or an unexplained high level: consultation, because liver, kidney and blood conditions can raise it.
Smokers: B12 only for a measured low or borderline level, a vegan diet, metformin or a long-term acid blocker; never more than 1,000 micrograms a day.
Advanced kidney disease: methylcobalamin or hydroxocobalamin is preferred; for a physician.
Leber hereditary optic neuropathy: no cyanocobalamin.
Pregnancy and breastfeeding: safe at supplement doses.
Under 18: outside the Supplement Check.

Dose and timing

Shortage or high risk: 1,000 micrograms once a day by mouth.
Vegans: 250 to 500 micrograms a day, or 2,000 micrograms once a week (common practice).
On metformin, or 2 years or more on an acid blocker: a B12 blood test first; no tablet until the result is in.
A borderline level (150 to 258 pmol/L): a methylmalonic acid or active B12 blood test decides.
Over 50 with no cause: B12 from fortified plant drinks or cereals, or from meat, fish, eggs and dairy.
How to tell it is working: a repeat B12 test at 2 to 3 months (with haemoglobin if you were anaemic), and your symptoms.
Store at room temperature, away from light.

What we know about Canadians

Most Canadians have enough B12. In the national blood survey (2012 to 2019), 3.5% of adults were in the deficient range (under 148 pmol/L) and another 17.8% in the borderline range (148 to 220 pmol/L); homocysteine, a sign the body is short of B12 or folate, was high in 6% and rose with age. [11] The lowest levels were in people who ate little red meat, milk or fortified plant drinks and eggs, and in those who took no supplement. [11] Canadian surveys have not measured B12 in metformin or acid-blocker users separately; the figures for those medicines come from US studies. [4, 5]

Most at risk: adults over 60, vegans and people who eat little meat, dairy or eggs, metformin users (risk rises with years), people on acid blockers for 2 years or more, and people after weight-loss surgery.

How sure are we: moderate–high. National blood survey with measured levels, 2012 to 2019; the medicine effects come from US studies (moderate).

An honest comparison: correcting a low B12

The same goal, with the cost a month in Canadian dollars.

Option	Effect	How sure we are	Cost a month (CAD)
B12 1,000 micrograms a day by mouth	Blood levels similar to injections in 3 small trials [6]	Low	$3 to $5
B12 on age alone, no shortage	No benefit for nerves, memory or thinking [1, 2]	Moderate	$3 to $5, spent without benefit
Food first: fortified plant drinks and cereals, or meat, fish, eggs and dairy	Prevents a shortage when absorption is normal	Moderate	Part of your food
B12 injections	The standard for a severe shortage or nerve signs	High (established practice)	$5 to $10, plus injection visits

Our verdict

B12 when a blood test shows a low or borderline level, or when there is a clear cause: a vegan diet, metformin, 2 years or more on an acid blocker, or weight-loss surgery. For metformin and acid blockers, the blood test comes first.

Not on age alone. Over 50 with no cause, get B12 from fortified foods or animal foods.

One Canadian adult in five has a low or borderline B12, so the question matters. But the one well-run trial in older people with moderately low levels found no benefit from a year of B12, and B12 does nothing measurable for memory, mood or energy without a shortage. Selling it to everyone over 50 would sell a product without a need. Dr. French's ruling: a B12 blood test for anyone on metformin or on an acid blocker for 2 years or more, and a tablet for those who are low. Tablets by mouth work as well as injections for most people and cost a few dollars a month.