Iron tablets are among the most commonly prescribed and self-bought supplements, and they are also among the most likely to cause digestive side effects. Most are manageable with a change in how and when you take them.
One safety point comes first: iron is a leading cause of serious accidental poisoning in young children. Tablets can look like sweets, and a small number can be dangerous to a small child. Keep iron in child-resistant packaging, well out of reach and sight. If a child may have swallowed any, go to an emergency unit immediately, even if they appear completely well, and take the packaging with you.
Common side effects reported with this supplement or ingredient
- Constipation, the most common complaint. See iron tablets and constipation.
- Nausea and stomach discomfort, especially on an empty stomach. See iron tablets and nausea.
- Diarrhoea in some people rather than constipation
- A metallic taste
- Dark or black stools
- Staining of the teeth with liquid preparations
Black stools on iron are expected and harmless, caused by unabsorbed iron. There is an important distinction, though: stools that are black and sticky or tarry, particularly with abdominal pain, dizziness, or vomiting blood, can indicate bleeding in the gut and need urgent assessment. Iron can make that harder to spot, so do not assume a change is just the tablets if you feel unwell with it.
Digestive effects are dose-related. Taking iron with a little food, or every second day rather than daily, is often better tolerated and in some cases absorbs just as well. Ask your prescriber before changing the schedule.
Possible interactions with medicines, chronic conditions or pregnancy
Iron both blocks and is blocked by a number of things:
- Levothyroxine. Iron reduces its absorption; separate them by several hours. See levothyroxine side effects.
- Some antibiotics, including tetracyclines and quinolones
- Antacids and stomach acid medicines, which reduce iron absorption
- Calcium supplements and dairy, also best separated
- Tea and coffee, which noticeably reduce absorption if taken with the dose
Vitamin C, or a glass of orange juice, improves absorption and is often suggested alongside.
Iron is commonly recommended in pregnancy, but at an appropriate amount rather than a self-chosen one. See iron tablets in pregnancy.
Who should speak to a healthcare professional before using it?
Anyone taking iron without a confirmed reason. Iron deficiency should be diagnosed on a blood test, not assumed from tiredness, and the cause matters as much as the treatment: in adults, unexplained iron deficiency sometimes points to bleeding that needs investigating.
Also ask first if you have a condition causing iron overload such as haemochromatosis, inflammatory bowel disease, or if you have had stomach or bowel surgery.
Dose, product-quality and stacking risks
Supplements in South Africa are regulated as complementary medicines, and the checks applied are not the same as those for prescription medicines. Strength, purity and labelling can vary between products. See how supplements are regulated in South Africa and how to check supplement labels.
Stacking is a common problem: several products each containing the same ingredient add up to more than intended. Read the ingredients on everything you take. See taking too many supplements.
Iron appears in multivitamins, prenatal formulas and tonics as well as dedicated tablets, and different salts contain different amounts of actual elemental iron, so two products labelled with the same number are not necessarily equivalent. A pharmacist can tell you what you are actually taking.
When to stop and get medical advice
Emergency care for any suspected overdose, particularly in a child, and for black tarry stools or vomiting blood.
Speak to a pharmacist or doctor if constipation or nausea is severe enough that you are skipping doses, since a different preparation or schedule usually solves it, and untreated deficiency is worth avoiding.
Related supplement safety guides
- Supplement side effects
- Supplement and medicine interactions
- When to see a doctor about a supplement
- Unsafe supplements in South Africa
- Report a supplement side effect in South Africa
Medical Disclaimer: This content is for informational and educational purposes only and does not constitute medical diagnosis, prescription, or professional advice. Always read the patient information leaflet and consult a qualified doctor or pharmacist regarding any medicine or health concern.