Iron & Vitamin B12 in GI Disease

Mineral and vitamin supplements

Iron tablets or liquid, intravenous (IV) iron, vitamin B12 tablets or injections

Iron is needed to make haemoglobin, the part of red blood cells that carries oxygen. Vitamin B12 is needed for blood cells and nerves. In gut disease you can lose iron through bleeding or fail to absorb it (for example in coeliac disease or inflammatory bowel disease), and B12 is absorbed at the end of the small intestine and needs stomach factors, so it can run low after stomach or ileal surgery, in Crohn’s disease affecting the ileum, in atrophic gastritis or with some long-term acid-reducing medicines. Replacing what is missing corrects anaemia and symptoms, but it is also important to find and treat the cause.

Do not start iron without a blood test: too much iron can be harmful, and iron can hide a serious bleeding problem. Your doctor chooses the form and dose. Iron tablets are usually best absorbed on an empty stomach or with vitamin C, but they can be taken with a little food if they upset the stomach; avoid taking them with tea, coffee, milk, calcium or antacids. Some doctors suggest taking iron once a day or on alternate days. Keep iron out of children’s reach: an overdose is dangerous for a child. If tablets do not work or are not tolerated (for example in active IBD or after some surgeries), iron can be given into a vein. Vitamin B12 can be given as tablets or as injections under the skin or into a muscle; injections are usually used when absorption is poor.

• Swelling of the face, lips or throat, wheeze, or trouble breathing during or soon after an iron infusion or injection — Could be a serious allergic reaction. Tell staff immediately; if at home, call emergency services now.

• Black, tarry, sticky and foul-smelling stools, or vomiting blood — Iron makes stools dark, but tarry sticky stools or any blood vomiting suggest bleeding from the upper gut. Go to the emergency department.

• Chest pain, fainting, severe breathlessness or a racing heart with anaemia — Severe anaemia can strain the heart. Go to the emergency department.

• Numbness, tingling, balance problems, memory change or marked tiredness despite treatment (B12) — Could mean B12 deficiency is affecting nerves or the dose or route is not enough. See your doctor promptly rather than waiting.

• Accidental swallowing of iron tablets by a child — A medical emergency, even if the child seems well. Go to the emergency department immediately; do not induce vomiting.

Iron absorption is reduced by antacids, acid-lowering medicines (proton pump inhibitors), calcium, tea, coffee, milk and some antibiotics, and iron can reduce absorption of some medicines such as thyroid tablets and certain antibiotics; ask your pharmacist how far apart to take them. Metformin and long-term acid-lowering medicines can lower B12 over time. Always tell your doctor what you take.

Blood tests (blood count, ferritin and other iron tests, and B12) are repeated after some weeks to months to check the response, and then from time to time. Your doctor will also look for why you became deficient, which may include tests such as endoscopy or colonoscopy, especially in adults with iron deficiency.

Iron is usually continued for some time after the blood count normalises, to refill the body’s stores; your doctor will tell you when to stop. B12 may be needed for life if the cause cannot be corrected. Do not stop or restart on your own.

Iron and B12 needs rise in pregnancy and both are commonly used, but doses must be set by your doctor or midwife. Do not take high-dose supplements without advice.

Iron and B12 deficiency are common reasons for tiredness, and tea or coffee drunk with meals (including around iftar and suhoor) can reduce iron absorption; leave about an hour or two between them and your iron tablet if your doctor agrees.

• Why am I low in iron or B12, and do I need tests to find the cause?

• Should I take tablets, injections or an infusion?

• How can I reduce constipation or stomach upset from iron?

• When will you repeat my blood tests?

• How long will I need treatment?

This guide comes from the Saudi Gastroenterology Association (SGA) patient library, prepared by the Approved for publication by SGA; individual clinician review is ongoing. Last reviewed: 2026-04-24. It is for education and does not replace your doctor's advice.