Iron Deficiency and Canker Sores — Why Ferritin Matters More Than Hemoglobin

TL;DR

People with recurring canker sores are more likely to have low iron stores. A 2015 meta-analysis of nine studies found they had about 2.6 times the odds of ferritin deficiency compared with people who don't get canker sores (Chen et al., 2015 — PMID: 25720915). In a classic 1975 study, about 18% of canker sore patients had a deficiency of iron, folate, or B12, and correcting it put many of them into remission (Wray et al., 1975 — PMID: 1148667). Not every study agrees, and iron seems to matter less than folate or B12. The key is the test: ask for ferritin, which measures your iron stores. Hemoglobin, the number on a routine blood count, only drops after your stores have been low for a while, so it's possible to be iron deficient with a normal blood count. Only take iron if testing shows you're low. If you are, taking it every other day is absorbed better than taking it daily.

What the Research Shows

The evidence for a link

The first good evidence came from a 1975 study in the BMJ. Researchers tested 130 consecutive patients with recurring canker sores and found that about 18% had a deficiency of iron, folate, or vitamin B12, compared with about 8% of matched controls. Iron was the most common: 15 of the 23 deficient patients were low on iron. When the deficiencies were corrected, 15 patients went into full remission and 8 more improved (Wray et al., 1975 — PMID: 1148667).

Later studies have mostly supported this:

• In 32 patients and 29 controls, 56% of patients had a deficiency of iron, folate, or B12, compared with 7% of controls (Compilato et al., 2010 — PMID: 19888941).
• A 2015 meta-analysis pooling nine studies found people with canker sores had about 2.6 times the odds of ferritin deficiency (Chen et al., 2015 — PMID: 25720915). A 2024 meta-analysis found almost the same thing for ferritin, but no significant difference in serum iron (Mousavi et al., 2024 — BMC Oral Health). That makes sense: ferritin reflects your stores, while serum iron bounces around depending on what you ate recently.

Where the evidence is weaker

Not every study finds a difference. One study of 35 patients and 26 controls found lower B12 in the canker sore group but no difference in iron or ferritin (Piskin et al., 2002 — J Eur Acad Dermatol Venereol).

And iron seems to be the weaker player among the deficiencies. In the 1975 study, iron-deficient patients improved less dramatically with treatment than those low in B12 or folate. In the 2015 meta-analysis, folate deficiency had a much stronger association than ferritin.

The fair summary: low iron stores are linked to canker sores and worth checking for, but correcting them won't fix everyone, and you should check B12 and folate at the same time.

Why Iron Would Matter

The cells lining your mouth divide quickly and use a lot of energy. Iron is built into the enzymes cells use to produce that energy and to copy DNA when they divide. When iron runs short, fast-dividing tissues are among the first to suffer. That's why iron deficiency also causes a sore, smooth tongue, cracks at the corners of the mouth, and brittle nails, often before anemia shows up.

Iron also matters for the immune system, and canker sores happen when immune cells attack the lining of the mouth. How exactly low iron makes that attack more likely hasn't been mapped out.

The rough picture: low iron leaves the lining of your mouth thinner and slower to repair, so the minor scrapes and irritations that start canker sores are more likely to turn into one.

Ferritin vs. Hemoglobin: The Test That Matters

This is where a lot of people get missed.

When your body runs short of iron, it uses up its stores first, and it protects your red blood cells for as long as it can. So iron deficiency happens in stages:

1. Stores drop. Ferritin falls. Hemoglobin is still normal.
2. Stores run out. Ferritin is very low. Hemoglobin is still often normal or borderline.
3. Anemia. Only now does hemoglobin drop below normal.

A routine blood count (CBC) measures hemoglobin. So if that's the only test you get, you'll come back "normal" through the first two stages, even though your iron stores are depleted. Ferritin catches it much earlier.

What to ask for: serum ferritin, by name. If your doctor is already ordering blood work, ask them to add ferritin, B12, and folate.

What the numbers mean

Ferritin cutoffs are less settled than you'd expect:

• Under 15 µg/L (ng/mL): The World Health Organization's cutoff for iron deficiency in otherwise healthy adults (WHO, 2020).
• Under about 30: Many clinicians use a higher cutoff than WHO's, and newer research suggests the real threshold where iron starts running short is probably above 15, somewhere around 25.
• If you have inflammation or an infection, ferritin rises artificially, because it's also an inflammation marker. WHO uses a cutoff of 70 in that situation. That's important for people with Crohn's, celiac disease, or another inflammatory condition, whose ferritin can look "normal" while their iron is actually low.

Lab reference ranges often start at 10–15 for women, so a result of 18 may be flagged as normal. If you get frequent canker sores and your ferritin is in the low 20s or below, that's worth bringing up with your doctor even if it isn't flagged.

Who's at Higher Risk

Heavy periods. This is the most common cause of iron deficiency in women before menopause. Monthly blood loss can drain iron faster than diet replaces it, and many women with heavy periods have low ferritin for years without being told. See Canker Sores During Your Period.

Pregnancy. Iron needs climb steeply, and iron deficiency is the most common nutritional deficiency in pregnancy. See Canker Sore Treatments During Pregnancy.

Plant-based diets. Plant iron (non-heme iron) is absorbed much less efficiently than the heme iron in meat. A vegetarian eating plenty of iron-rich foods can still come up short. See Vegan and Vegetarian Canker Sores.

Celiac disease. Iron is absorbed at the top of the small intestine, which is the part celiac disease damages. Unexplained iron deficiency is one of the classic signs of undiagnosed celiac. See Gluten and Canker Sores.

Crohn's disease and ulcerative colitis. Both cause iron loss through bleeding and poor absorption. About a quarter of people with Crohn's have anemia, and more than half of those are iron deficient. See Canker Sores and Crohn's Disease.

Regular blood donors. Each donation takes iron with it.

Regular NSAID use. Frequent ibuprofen, naproxen, or aspirin can cause slow, unnoticed bleeding in the gut. NSAIDs may also trigger canker sores directly. See What Causes Canker Sores.

Long-term acid reducers. Stomach acid helps you absorb iron, so years of proton pump inhibitors like omeprazole can reduce absorption.

In men and in women after menopause, iron deficiency without an obvious cause needs a medical workup, because it can be a sign of bleeding somewhere in the gut. Don't just treat it with supplements and move on.

Getting Iron From Food

Heme iron (from animal foods)

Heme iron is absorbed much better than plant iron, and other foods in the meal don't affect it much. Good sources:

• Red meat (beef, lamb)
• Liver and other organ meats, which are very high in iron
• Dark-meat poultry
• Clams, mussels, oysters, and sardines

Non-heme iron (from plant foods)

Lentils, beans, chickpeas, tofu, spinach, pumpkin seeds, and fortified cereals all contain iron, but you absorb much less of it, and what you eat alongside it makes a big difference.

Things that help: vitamin C from food at the same meal (tomatoes, peppers, citrus, broccoli). Eating even a small amount of meat or fish with plant foods also increases how much plant iron you absorb.

Things that hurt: tea and coffee with the meal (their tannins block iron absorption, so have them between meals instead), calcium supplements or a lot of dairy at the same meal, and the phytates in unsoaked grains and legumes.

For a full eating plan, see Diet for Canker Sore Prevention.

Taking Iron Supplements

Don't take iron unless you're low

This is different from B12. Your body has no good way to get rid of extra iron, so it builds up. People with hemochromatosis, a common genetic condition, absorb too much iron to begin with, and supplements can push them into organ damage. Iron pills are also a well-known cause of serious poisoning in young children, so keep them out of reach.

If your ferritin is normal, iron supplements won't help your canker sores and can cause harm.

Every other day beats every day

This is newer research and many people (and some doctors) haven't caught up with it.

When you take iron, your body releases a hormone called hepcidin that temporarily blocks further iron absorption for about a day. So if you take iron every morning, each dose is partly blocked by the hepcidin from the day before. In a study of iron-depleted women, taking iron every other day led to higher absorption per dose and more total iron absorbed over the study period than taking it every day (Stoffel et al., 2017 — PMID: 29032957). Splitting the dose into two smaller doses a day didn't improve absorption and pushed hepcidin higher.

The practical takeaway: one dose every other day, in the morning, on an empty stomach if you can tolerate it. Your doctor may still prefer daily dosing for severe deficiency. It's worth asking about.

Vitamin C with your pill probably doesn't matter

The old advice is to take iron with orange juice or a vitamin C tablet. A randomized trial of 440 adults with iron-deficiency anemia found that adding vitamin C to iron supplements didn't raise hemoglobin any faster than iron alone (Li et al., 2020 — JAMA Netw Open). Vitamin C still helps you absorb plant iron from food. With a supplement, it doesn't seem to add much.

Choosing a form

• Ferrous sulfate is the cheapest and most studied, but it's the most likely to cause stomach upset and constipation.
• Ferrous gluconate has less iron per tablet and is a bit gentler.
• Iron bisglycinate (sold as "gentle iron") is generally easier on the stomach.

Whatever form you choose, side effects are the main reason people stop. Every-other-day dosing often helps with that too.

If testing shows you're low and your doctor agrees, iron bisglycinate is a reasonable form to start with:

IV iron

If oral iron isn't absorbed (celiac disease, active Crohn's, bariatric surgery), isn't tolerated, or isn't working fast enough, iron can be given by IV. It bypasses the gut completely and refills stores quickly. A gastroenterologist or hematologist usually arranges it.

Recheck

Recheck ferritin after about three months. It often takes several months of supplements to rebuild stores fully, even after hemoglobin has recovered, and stopping too early is a common reason deficiency comes back.

While You Work on Your Iron

Correcting a deficiency takes months. In the meantime, sores still need managing. A barrier patch protects an active sore from food and your tongue: