Dark or black poo after taking iron tablets: what is normal and when to worry

If you have started iron tablets and your poo has become dark green, dark brown or black, that is a recognised effect of the medicine and, on its own, is not a sign that something is wrong. The NHS lists darker than usual poo as a common side effect of ferrous fumarate and of iron tablets prescribed for iron deficiency anaemia.

The caution is that black poo has another, more serious meaning. Blood that has been digested on its way through the bowel can turn stool black and tarry. Because the tablets can mask or mimic this, the features that separate the two matter more than the colour alone: texture, smell, stickiness, how you feel, and whether the colour changed when you started the iron.

Iron that is not absorbed in the gut passes through into the stool, where it can darken it. Guts UK describes black poo as something that can happen if you take iron tablets or a bismuth-based medicine, or have eaten a lot of liquorice, and says it should not cause a problem if the colour is the medicine effect. It adds that this type of poo is often hard and not sticky.

Other effects of iron are worth knowing about because they often come together. The NHS lists constipation or diarrhoea, tummy pain, heartburn, and feeling sick among the common side effects, and suggests taking tablets with or soon after food to reduce them. Some people take ferrous fumarate on alternate days if it upsets the stomach, which the NHS describes as an option when once-daily dosing causes side effects.

Do not stop prescribed iron because of the colour change alone. The NHS says it is important to keep taking the tablets and that you can ask a pharmacist or doctor for advice if side effects bother you or do not go away.

Melaena is the medical term for black, tarry stool caused by bleeding in the upper digestive tract, such as from a stomach ulcer. The features below compare the two, using the descriptions given by the NHS and Guts UK.

The NHS stomach ulcer advice sets out what needs immediate action, and the same signs apply if you are on iron and have something that does not look like a simple medicine effect.

• Call 999 or go to A&E if your poo is black, sticky and very smelly or has blood in it, you vomit bright red blood or vomit that looks like ground coffee, you have severe tummy pain or a tummy that is painful to touch, or you have sudden chest pain.

• Ask for an urgent GP appointment or call NHS 111 if tummy pain is getting worse, you feel full after very little food, you have lost your appetite or weight without trying, you are tired all the time or you keep being sick.

• If your poo has turned black and you are not taking iron, bismuth or eating liquorice, Guts UK says to see your doctor, and to contact the NHS urgently if it is black, tarry and smells bad, especially with tummy pain.

• Cancer Research UK adds that blood from higher in the bowel may look dark red or black and can make poo look like tar, and that this can be from a bleeding stomach ulcer or from cancer higher up the bowel, so any such bleeding should be checked.

Iron tablets are usually prescribed for iron deficiency anaemia. The NHS says the causes include heavy periods, pregnancy, and bleeding in the stomach and intestines, which can come from anti-inflammatory painkillers such as ibuprofen and aspirin, stomach ulcers, inflammation of the bowel or food pipe, piles, and less commonly cancers of the bowel or stomach. It says that if the reason for the anaemia is not clear a GP may arrange a faecal immunochemical test (FIT) and may refer you to a specialist.

If you are taking iron without a clear explanation for your low iron level, or the level does not rise as expected after treatment, it is reasonable to ask your GP whether the cause has been looked for. The NHS says the GP may carry out repeat blood tests over the next few months to check that your iron level is getting back to normal, and that tablets are usually needed for about 6 months.

For iron bought over the counter, the NHS advises not taking ferrous fumarate for longer than 6 months without a prescription and seeing a GP if symptoms get worse or do not get better.

The NHS suggests taking ferrous fumarate at the same time each day, with water, and notes that taking it with food may help stomach side effects, although some foods and drinks such as tea, eggs or milk can affect how well it works. It also lists medicines that can interact, including some antibiotics, indigestion medicines, other supplements containing calcium, zinc or magnesium, and levothyroxine, so tell a pharmacist what else you take.

Iron overdose is dangerous for children. The NHS says to keep iron tablets out of reach of children and to call NHS 111 straight away if you or a child have taken more than the dose on the packet, leaflet or prescription. Bring the packet to A&E if you are told to go.

If constipation from iron is troublesome, the NHS constipation advice includes more fibre and fluid, regular toilet routine and activity, and a pharmacist can suggest a short course of laxative.