Vitamin B12 for Canker Sores — The Supplement With Real Trial Evidence

TL;DR

Vitamin B12 has the best evidence of any supplement for preventing canker sores. In a placebo-controlled trial, people with recurring canker sores took 1,000 mcg of B12 under the tongue every night for six months. By the last month, 74% of the B12 group were ulcer-free, compared with 32% on placebo, and the benefit showed up regardless of people's starting B12 levels (Volkov et al., 2009 — PMID: 19124628). People with canker sores are also more likely to be low on B12 to begin with (Mousavi et al., 2024 — BMC Oral Health). B12 is very safe at this dose. Two things that matter less than you'd think: whether you take it under the tongue or swallow it (they produce similar blood levels), and whether it's methylcobalamin or cyanocobalamin (there's no good evidence one works better). Give it at least three to six months, and if you can, get tested first, especially if you take metformin or an acid reducer.

What the Trial Found

The study everyone cites, for good reason, is Volkov et al. (2009), published in the Journal of the American Board of Family Medicine.

• Who: 58 people with recurring canker sores completed the trial
• What: 1,000 mcg of vitamin B12 under the tongue at bedtime, or a placebo, for six months
• Results: By months five and six, the B12 group had shorter outbreaks, fewer ulcers, and less pain than the placebo group. In the final month, 74% of the B12 group had no ulcers at all, compared with 32% of the placebo group.
• The important part: The benefit appeared regardless of people's B12 levels at the start (Volkov et al., 2009 — PMID: 19124628).

That last finding is unusual. For most supplements, like iron or zinc, the benefit is concentrated in people who are deficient. Here, people with normal blood levels seemed to benefit too. A smaller, earlier study came to a similar conclusion, reporting that B12 may help canker sores even when blood levels are normal (Gulcan et al., 2008 — PMID: 19011392), though it wasn't as rigorously designed.

Two caveats. It's one solid trial, not a large body of them. And the benefit took months to appear: the differences were clearest in the fifth and sixth months. So if you try B12, give it time.

People With Canker Sores Are Often Low on B12

Separately from the trial, people with recurring canker sores are more likely to have low B12 than people without them. A 2024 meta-analysis pooling many studies found they had about three times the odds of B12 deficiency (Mousavi et al., 2024 — BMC Oral Health).

Why B12 Matters for Your Mouth

The lining of your mouth replaces itself every couple of weeks, which means its cells are dividing constantly. B12 is essential for making DNA when cells divide. When B12 runs short, fast-dividing tissues struggle first, which is why B12 deficiency also causes a sore, smooth, red tongue and cracks at the corners of the mouth.

B12 also matters for nerves and for the immune system. How exactly it reduces canker sores, especially in people who aren't deficient, hasn't been worked out. It's possible that blood tests miss a lot of people whose tissues are running short (see the testing section below), or that high-dose B12 has effects beyond correcting a deficiency.

Who's Most Likely to Be Low

People who don't eat animal foods. B12 is found almost only in animal products. Vegans need a supplement or fortified foods, period, and many vegetarians do too. See Vegan and Vegetarian Canker Sores.

People taking metformin. In a long-term diabetes prevention study, people taking metformin were about twice as likely to have low B12 after five years as those on placebo, and the risk grew the longer they took it (Aroda et al., 2016 — J Clin Endocrinol Metab). If you take metformin and get canker sores, ask for a B12 test.

People taking acid reducers long-term. You need stomach acid to free B12 from food. In a study of more than 25,000 people with B12 deficiency, two or more years of a proton pump inhibitor (omeprazole, esomeprazole, and similar) was linked to about 65% higher odds of deficiency, and H2 blockers (famotidine and similar) to about 25% higher odds (Lam et al., 2013 — PMID: 24327038).

Adults over 50. Stomach acid production tends to drop with age, which makes it harder to absorb B12 from food.

People with gut conditions or surgery. B12 is absorbed at the end of the small intestine with the help of a protein made in the stomach (intrinsic factor). Pernicious anemia (an autoimmune condition that destroys the cells making intrinsic factor), celiac disease, Crohn's disease, weight-loss surgery, and surgery on the stomach or small intestine can all cause deficiency. See Intrinsic Factor and Canker Sores.

People who use nitrous oxide. Nitrous oxide (laughing gas, including recreational "whippets") inactivates B12 in the body. Heavy recreational use can cause serious deficiency and nerve damage.

Testing: Why "Normal" Isn't Always Normal

The standard test is serum B12. It's useful, but it's a fairly late and imprecise marker: blood levels can look normal while your tissues are already running short (PMID: 19623286).

Two other tests can catch it earlier:

• Methylmalonic acid (MMA). This builds up when your body doesn't have enough B12 to do its job. A high MMA with a borderline B12 suggests real deficiency.
• Holotranscobalamin (active B12). This measures the portion of B12 that's actually available to your cells. It isn't offered everywhere.

Labs differ, but a B12 result in the low-normal range in someone with symptoms is worth following up with an MMA test.

Do you need to test before taking B12? Not strictly. B12 is very safe, and the trial found benefit even in people with normal levels. But testing is still worth doing, because a real deficiency can have a cause (like pernicious anemia or celiac disease) that needs its own treatment, and severe deficiency may need injections rather than tablets. If you're going to be tested, have it done before you start supplementing, since supplements will make the result look normal.

Under the Tongue or Swallowed?

The Volkov trial used B12 under the tongue (sublingual), and a lot of supplement marketing claims this route "bypasses" the gut. The evidence doesn't really support that.

In a direct comparison, people taking 500 mcg of B12 either under the tongue or swallowed ended up with nearly identical blood levels after four weeks (Sharabi et al., 2003 — PMID: 14616423). The reason high-dose B12 works even for people with absorption problems isn't the route. It's the dose. Normally B12 is absorbed through a specific pathway that can only handle small amounts at a time. But about 1% of a large dose is also absorbed passively, without that pathway. One percent of 1,000 mcg is 10 mcg, which is several times the daily requirement.

That's also why high-dose oral B12 works about as well as injections for many people with deficiency. A Cochrane review found that oral doses of 1,000–2,000 mcg raised B12 levels similarly to injections, though the evidence was limited (Wang et al., 2018 — PMID: 29543316). People with severe deficiency, nerve symptoms, or major absorption problems may still need injections, so that's a conversation with your doctor.

The practical takeaway: if you like dissolving it under your tongue, that's what the trial did, so go ahead. But a swallowed tablet at the same dose is fine.

Methylcobalamin or Cyanocobalamin?

You'll see methylcobalamin marketed as the "active" or "better" form. There's no good evidence it works better than cyanocobalamin, the form in most cheaper supplements. Your body converts both into the forms it uses, and most B12 trials, including many of the ones that established B12 treatment, used cyanocobalamin.

Pick whichever you'll actually take consistently. Methylcobalamin is fine, and so is cyanocobalamin. What matters is the dose (1,000 mcg was used in the trial) and taking it every day.

If you also want folate, which is the other nutrient most strongly linked to canker sores, a combined product covers both:

See Folate and Canker Sores for why folate matters and why you shouldn't take high-dose folate without making sure your B12 is covered.

Is High-Dose B12 Safe?

Yes. There's no established upper limit for B12, because no level of intake has been shown to cause harm in healthy people. Extra B12 is mostly excreted in urine. A few people notice acne flare-ups on high-dose B12, and anyone with kidney disease should check with their doctor before taking high-dose supplements of any kind.

How to Try It

• Dose: 1,000 mcg a day, as in the trial
• Timing: Bedtime under the tongue matches the trial, but any time of day is fine
• Duration: At least three months, ideally six. The trial's differences were clearest after five months.
• Track it: Note the date of every new sore for a few months before you start and while you're taking it. Otherwise you won't know whether it helped.
• Test first if you can, especially if you're vegan, take metformin or an acid reducer, are over 50, or have a gut condition

If you're truly deficient, you may notice more energy or less tingling in your hands and feet before you notice fewer canker sores. Those systems often respond faster.

Food Sources

B12 is found almost only in animal foods:

• Clams, mussels, and oysters: the richest sources by far
• Liver: extremely high
• Fish such as salmon, tuna, trout, and sardines
• Beef and lamb
• Eggs and dairy: smaller amounts, but they add up
• Fortified foods: some cereals, plant milks, and nutritional yeast

For people who eat animal foods and absorb normally, diet usually covers B12. For vegans, people on metformin or long-term acid reducers, and people with absorption problems, a supplement is the reliable option. See Diet for Canker Sore Prevention.