Zinc for Canker Sores — The Evidence, Best Form, and How to Test

TL;DR

Zinc is one of the supplements worth taking seriously for canker sores, but only if you're low. A 2021 meta-analysis of 19 studies found that people with recurring canker sores have lower blood zinc on average than people without them (Al-Maweri et al., 2021 — Clin Oral Investig). In a placebo-controlled trial of 40 patients, a month of zinc sulfate reduced ulcers, and they stayed down for three months afterward (Orbak et al., 2003 — PMID: 12790293). In an older study, people with lower starting zinc levels were the ones who improved (Merchant et al., 1977 — PMID: 870981). But a double-blind trial that gave zinc to people without checking their levels first found no benefit, and its authors recommended against taking zinc blind (Wray, 1982 — PMID: 7048184). So get your zinc tested before you supplement. If it's low, 25–30 mg of elemental zinc a day with food is a sensible dose. Don't go above 40 mg a day long-term, because too much zinc causes copper deficiency.

Why Zinc Matters for Your Mouth

The lining of your mouth replaces itself constantly, roughly every couple of weeks. The cells doing that work divide fast, and fast-dividing cells need zinc. It's involved in making new DNA, in the enzymes that build tissue, and in wound repair generally. This is why zinc deficiency shows up early in fast-turnover tissues like skin, the gut lining, and the mouth.

Zinc also matters to the immune system, which is the other half of the canker sore story. Canker sores happen when immune cells attack the cells lining the mouth, and zinc deficiency is known to throw immune regulation off in a number of ways. Exactly how that connects to canker sores hasn't been worked out, but it's a reasonable place for the link to sit.

Put those together and it makes sense that low zinc would make you more prone to sores, and slower to recover from them. What the research supports is narrower than that, though, so it's worth going through.

What the Studies Found

People with canker sores tend to have lower zinc

A 2021 meta-analysis pooled 19 studies comparing blood zinc levels in about 1,000 people with recurring canker sores and about 950 people without. The canker sore group had meaningfully lower zinc on average (Al-Maweri et al., 2021 — Clin Oral Investig).

One caveat: the studies varied a lot from one another. Some found a big difference and some found little. That usually means the relationship depends on the population. Zinc deficiency is more common in some countries and diets than others, so in some groups it's likely a big factor and in others a small one.

Zinc helps people who are low

The positive trials share a pattern: the benefit is concentrated in people with low zinc.

• Merchant et al., 1977: In a small study of 17 people given zinc sulfate, everyone whose starting zinc level was at or below a certain level improved, with 50–100% fewer episodes. Among people whose zinc was higher to begin with, only 3 of 8 improved (Merchant et al., 1977 — PMID: 870981). There was no placebo group, so it's suggestive rather than conclusive.
• Orbak et al., 2003: 40 people with recurring canker sores were randomized to zinc sulfate (220 mg a day, which is about 50 mg of elemental zinc) or placebo for one month. The zinc group had fewer ulcers, and the improvement held for the three months they were followed afterward. About 42% of the participants had low zinc at the start (Orbak et al., 2003 — PMID: 12790293).

A 2021 review of zinc supplementation trials found that 5 of the 7 studies reported fewer recurrences with zinc (Halboub et al., 2021 — J Trace Elem Med Biol).

Zinc for everyone doesn't seem to work

The most important negative result comes from a 1982 double-blind crossover trial in 25 patients. Everyone got zinc for three months and placebo for three months, without anyone checking their zinc levels first. Zinc did no better than placebo, four people dropped out because of side effects, and the author concluded that zinc shouldn't be given to canker sore patients without evidence of deficiency (Wray, 1982 — PMID: 7048184).

That fits with everything above. Zinc is a deficiency fix, not a general canker sore treatment. It's different from vitamin B12, where the main trial found benefit regardless of people's starting levels (see B12 for Canker Sores).

Topical Zinc

There's a separate question of whether putting zinc directly on a sore helps. One trial says it might. In a double-blind study of 46 people, a zinc sulfate tablet that sticks to the inside of the mouth made sores noticeably smaller by days 3, 5, and 7 compared with a placebo tablet, and reduced pain from day 4 onward (Ghorbani et al., 2020 — PMID: 32723352).

That product isn't widely sold in the US, and nobody has tested the zinc lozenges you'd find at a drugstore for canker sores. Cold-and-flu zinc lozenges often contain flavorings and citric acid that can sting an open sore. See Zinc Lozenges for Canker Sores for more on that.

Who's Likely to Be Low

You're more likely to be low on zinc if you:

• Eat little or no meat or seafood. The richest, best-absorbed sources of zinc are oysters, red meat, and poultry. Plant foods have zinc too, but they also contain phytates, compounds in whole grains and legumes that bind zinc and reduce how much you absorb. Soaking, sprouting, and fermenting help. See Vegan and Vegetarian Canker Sores.
• Have a gut condition that affects absorption, such as celiac disease or Crohn's disease, or chronic diarrhea.
• Drink heavily. Alcohol increases how much zinc you lose in urine.
• Take certain medications long-term, including some diuretics.
• Are older. Zinc absorption tends to drop with age.
• Are pregnant or breastfeeding, when needs go up.

If several of these apply and you get frequent canker sores, zinc moves up the list of things to check.

Testing for Zinc

The standard test is serum or plasma zinc, a basic blood test. It's not a perfect measure, and it helps to know the main ways it can mislead:

• Your body holds blood zinc steady even as stores drop, so a "normal" result doesn't completely rule out low zinc.
• It varies through the day and drops after meals. A morning, fasting blood draw gives the most consistent number.
• Illness and inflammation lower it. If you're sick or have an active inflammatory condition when you're tested, the result can look lower than your true status.

Labs use different reference ranges, so go by your lab's range. A result at or near the bottom of the range, in someone with frequent canker sores and a risk factor from the list above, is worth acting on.

Since you're getting blood drawn anyway, it makes sense to check the other nutrients linked to canker sores at the same time: B12, ferritin (iron stores), and folate. See Why Doctors Dismiss Canker Sores for how to ask for them.

How to Take Zinc If You're Low

Dose

25–30 mg of elemental zinc a day is a reasonable dose for correcting a deficiency, and it stays under the 40 mg daily upper limit set for adults by the NIH (NIH Office of Dietary Supplements). The Orbak trial used about 50 mg a day, but only for one month.

Labels can be confusing here. "Elemental zinc" is the amount of actual zinc. A tablet labeled "zinc gluconate 50 mg" contains much less than 50 mg of zinc. Most supplement labels now list the elemental amount in the Supplement Facts panel; that's the number to go by.

Form

• Zinc picolinate, zinc bisglycinate (glycinate), zinc citrate, zinc gluconate: all reasonable. The chelated forms (picolinate, bisglycinate) tend to be easier on the stomach.
• Zinc sulfate: the form used in most of the trials and it works, but it's the hardest on the stomach.
• Zinc oxide: cheap and common, but less well absorbed. Not the best choice for fixing a deficiency.

Timing

Take it with food. Zinc on an empty stomach makes a lot of people nauseous, and that's the main reason people quit. Keep it a couple of hours apart from iron supplements, which compete with zinc for absorption.

How long

Recheck your levels after about three months. If you're back in the normal range and your diet or absorption issue hasn't changed, you may need a lower ongoing dose. If the cause has been fixed (you've changed your diet, say), you may not need to keep supplementing at all.

The Copper Problem

This is the part most zinc articles leave out, and it's the reason "more is better" is a bad idea with zinc.

Zinc and copper compete with each other in the gut. When you take a lot of zinc for a long time, your intestinal cells make more of a protein that traps copper and keeps it from being absorbed. Over months, that can lead to copper deficiency, which causes anemia, low white blood cell counts, and nerve damage that can be permanent (Willis et al., 2005 — PMID: 15762288).

This is mostly a problem at higher doses taken for a long time, and it's the main reason the upper limit for adults is 40 mg a day. Some practical rules:

• Stay at or under 40 mg of elemental zinc a day unless a doctor is supervising.
• Count all your sources. Multivitamins, zinc lozenges, cold remedies, and some denture creams all contain zinc, and they add up.
• If you're going to supplement for more than a few months, recheck your levels and consider a product that includes a small amount of copper.
• Unexplained numbness, tingling, or balance problems while taking zinc are a reason to see a doctor promptly.

Getting Zinc From Food

If you'd rather fix things through diet, or want to keep levels up after correcting a deficiency:

• Oysters are in a category of their own. A 3 oz serving can contain several times the daily requirement, depending on the type.
• Beef, lamb, and pork: roughly 3–7 mg per 3 oz serving.
• Crab, lobster, and dark-meat poultry: good sources.
• Pumpkin seeds, cashews, hemp seeds: the best plant sources, though less is absorbed than from animal foods.
• Beans, lentils, chickpeas, whole grains: contribute zinc, but their phytates reduce absorption. Soaking and sprouting help.

For a full eating plan that covers zinc along with B12, iron, and folate, see Diet for Canker Sore Prevention.