What signs do B12 and iron deficiency cause in the mouth?

The upper surface of the tongue is covered with thousands of tiny projections called papillae; they give the tongue its rough texture and matt colour. This tissue is constantly renewing itself, and to do so it needs vitamin B12, folate and iron. MSD Manuals writes that B12, together with folate, is needed for cells to make DNA. With a deficiency, the papillae thin and disappear, and the tongue takes on a shiny, red, smooth appearance; this picture is called atrophic glossitis. A 2026 review from Poland stresses that nutritional deficiencies can affect the lining of the mouth before other symptoms appear, which is why the mouth is an important place for early diagnosis.

Among the symptoms of B12 or folate deficiency, alongside extreme tiredness and lack of energy, the NHS lists a sore, red tongue, mouth ulcers, pins and needles, muscle weakness, problems with vision, and problems with memory and thinking. The same page states plainly that some of these problems can occur without anaemia. With iron deficiency anaemia, among the less common symptoms the NHS lists a sore tongue, food tasting strange, difficulty swallowing, painful sores at the corners of the mouth, spoon-shaped nails and a craving to eat things that are not food, such as ice or paper.

The deficiency showing in the mouth without anaemia has also been shown in figures. In a study in China comparing 236 patients with atrophic glossitis with 208 healthy people matched for age and sex, B12 deficiency was found in 68.22% of patients and low ferritin in 13.98%; anaemia, however, was present in only 21.61%. In other words, most of these patients had no anaemia; a blood count alone was not enough to find the deficiency. In patients whose tongue had become completely smooth and whose atrophy covered more than half of the tongue surface, B12 deficiency and anaemia were more common; patients with B12 deficiency responded well to supplements.

Mouth symptoms also announce the deficiency within a picture of anaemia. In a retrospective study in Morocco of 34 patients diagnosed with B12 deficiency caused by an immune disease that targets the stomach lining (pernicious anaemia), 85.3% of patients had anaemia and 55.9% had mouth symptoms; in the 19 patients with mouth symptoms, the commonest were atrophic glossitis (7 patients) and a burning mouth (2 patients). The authors note that these changes in the mouth can appear without symptomatic anaemia, and that the dentist can play a part in the diagnosis.

The insidious side of B12 deficiency is its timing. MSD writes that the body stores B12 largely in the liver, that when intake stops these stores usually take 3 to 5 years to run out, and that anaemia may not develop until 3 to 5 years after the deficiency begins. Someone who switches to a vegan diet, has stomach surgery or has taken the same medicine for years may not connect the change in their tongue with that change, because there are years in between.

The commonest cause of deficiency is not a poor diet but a problem with absorption. MSD writes that B12 is absorbed in the last part of the small intestine after combining with a protein made in the stomach (intrinsic factor), and lists among the causes that disrupt absorption medicines that reduce stomach acid (antacids), metformin used in diabetes, coeliac disease, weight-loss surgery and the fall in stomach acid in old age. The NHS also lists epilepsy medicines and the stomach medicines called proton pump inhibitors among those that affect B12 absorption. According to the NHS, B12 and folate deficiency affects roughly 1 in 10 people aged 75 and over, and 1 in 20 people aged 65 to 74.

With iron deficiency, the real question is the cause. The NHS writes that iron deficiency anaemia is mostly due to blood loss or pregnancy, and that if the cause is not clear, the doctor may request further tests, such as a test for hidden blood in the stool, or refer you to a specialist. So a crack at the corner of the mouth or a sore tongue is the start not of taking iron tablets but of asking why the iron has run low. The NHS also lists bleeding from the stomach and bowel among the common causes: painkillers such as ibuprofen and aspirin, stomach ulcers, inflammation of the oesophagus or bowel, piles and, less often, stomach or bowel cancer. In someone who does not have periods, these possibilities should be discussed with a doctor.

Mouth ulcers are the most debated part of this subject. In a double-blind study in Israel of 58 people with recurring mouth ulcers, those who took 1,000 micrograms of B12 under the tongue every day for 6 months had significantly shorter outbreaks, fewer ulcers and less pain than with placebo in months 5 and 6; the effect was independent of the starting B12 level in the blood. By contrast, in another double-blind study of 160 people in the United States, a multivitamin containing the recommended daily amounts of vitamins did not change the number or duration of ulcer outbreaks over a year, and the authors said multivitamins should not be routinely recommended. The first study is small; its result should not be read as a firm treatment rule.

In the situations in the first list, it makes sense to ask your GP or dentist for a blood test; they are not urgent, but they should not be ignored. If any in the second list applies, see a doctor without delay.

The sequence below shows the route from a mouth symptom to finding the cause. It is your GP who will request the tests; your dentist can also pass the finding in your mouth on to them in writing.

1. 1

   Document the finding in your mouth

   Take a photo of your tongue, your ulcers or the crack at the corner of your mouth; for ulcers, note how many times a year they appear and how many days they last. If your dentist has seen atrophic glossitis or recurring ulcers, ask them to write it down.

2. 2

   Do not settle for a blood count

   Ask for B12, folate and ferritin levels in addition to a full blood count. MSD writes that B12 deficiency is mostly suspected when large red blood cells are seen on a blood count; but the study showing that most atrophic glossitis patients had no anaemia says this clue can be missed.

3. 3

   Tell the doctor about your medicines and diet

   Mention specifically any history of metformin, stomach medicines, epilepsy medicines, a vegan or restricted diet, stomach or weight-loss surgery, coeliac disease or inflammatory bowel disease. MSD also lists repeated use of laughing gas (nitrous oxide) among the causes.

4. 4

   If a deficiency is found, ask about the cause

   For B12 deficiency, MSD writes that in younger people further tests are done to find the cause, and sometimes an endoscopy to see the state of the stomach cells. For iron deficiency, the NHS states that if the cause is not clear, a test for hidden blood in the stool or a referral to a specialist may follow.

5. 5

   Have your mouth checked again after treatment

   A few months after treatment, show your dentist whether the papillae of the tongue have come back and whether the ulcers have become less frequent. If the mouth symptom is not improving, the cause should be looked for elsewhere.

Treating the mouth symptom means correcting the deficiency itself and its cause. The form, dose and length of supplements are set by the doctor according to your blood levels and the cause.

The points below are decisions that lead to a deficiency being missed or treated the wrong way.

The time depends on how severe the deficiency is and its cause. The course below is the general picture given in the sources; your doctor's follow-up plan takes priority.

1. The first weeks of treatment

   According to MSD, anaemia caused by B12 deficiency usually corrects itself in about 6 weeks. The sources give no clear time for how long the change in the tongue takes to recede; the study in Guangzhou reports that glossitis patients with B12 deficiency responded well to supplements.

2. The first months

   Folic acid treatment usually lasts 4 months, and iron treatment about 6 months. The NHS writes that a blood test may be repeated within a few months to see whether iron levels have returned to normal.

3. For mouth ulcers

   In the study of B12 under the tongue, the reduction in ulcers only became significant in months 5 and 6. If no result is seen within a few weeks, that may not mean the treatment is not working.

4. Nerve symptoms

   MSD writes that severe nerve symptoms that have lasted a long time can be permanent. How far numbness and pins and needles recover depends on how early treatment started.

We do not give a figure here, because behind the same mouth symptom there may be a mild deficiency that corrects itself with diet, or an absorption disorder that needs following for a long time. These are the items that determine the scope:

The tests requested

A blood count, B12, folate and ferritin are the basic set; depending on the cause, further blood tests, a test for hidden blood in the stool or an endoscopy may be added.

The cause of the deficiency

A deficiency due to diet may correct itself with short-term supplements, while an absorption disorder may need lifelong treatment.

The form of supplement

Tablets or injections, how often follow-up tests are done and how long treatment lasts vary according to the doctor's plan.