Iron Tablets and Your Poo: Dark Stool, Constipation

Started iron tablets and now your poo looks different? The NHS iron deficiency anaemia page says some people get constipation or diarrhoea, tummy pain, heartburn, feeling sick, or darker than usual poo from iron tablets. Darker poo is common with iron and usually nothing to worry about. Black, tar-like poo is a different matter, and the NHS treats it as one.

What the NHS lists, and what it suggests

The NHS page on ferrous sulfate side effects pairs the common side effects with something to try. These are the ones that involve your gut:

Side effect	NHS suggestion
Feeling or being sick	Take it with, or just after, a meal or snack; stick to simple meals, not rich or spicy food
Stomach discomfort or heartburn	Take it with or just after food; eat and drink slowly, have smaller and more frequent meals
Constipation	More fibre from fresh fruit, vegetables and cereals, plenty of water, more regular exercise such as a daily walk
Diarrhoea	Drink lots of fluids to avoid dehydration; don’t take other diarrhoea medicines without asking a pharmacist or doctor
Darker than usual poo	Common with iron and usually nothing to worry about

If the advice doesn’t help and a side effect keeps bothering you or doesn’t go away, the NHS says to speak to a doctor or pharmacist. Signs of dehydration, it says, include peeing less than usual or dark, strong-smelling pee. The NHS also notes that being sick, or severe diarrhoea lasting more than 24 hours, can stop contraceptive pills from protecting you, so check the pill packet.

Food helps the stomach but costs some absorption

The NHS guide on how to take ferrous sulfate says it works best on an empty stomach, ideally 30 minutes before eating or 2 hours after. If it upsets your stomach, you can take it with or after food. Tea, coffee, eggs and dairy can reduce how much iron gets in, so the NHS suggests leaving a 2-hour gap.

Which way suits you is a question for your doctor or pharmacist. The NHS iron deficiency anaemia page says to follow the GP’s advice on taking the tablets and that it’s important to keep taking them even if you get side effects. The dosing page adds that a doctor may suggest alternate days if side effects come up. The NHS common questions page says a side effect from one type of iron may not happen with another, such as ferrous fumarate or ferrous gluconate. Your doctor or pharmacist can say which applies to you.

Darker poo versus black, tar-like poo

The same ferrous sulfate page that calls darker poo common says to talk to a doctor or call 111 straight away if your poo is black, tar-like or red, has blood in or on it, or you feel unwell in any other way.

MedlinePlus lists iron pills among things that can make stool black, but says black or tarry stool with a foul smell most often means bleeding in the esophagus, stomach or first part of the small intestine. A provider can test the stool with a chemical to check for blood. The NHS iron deficiency anaemia page also names bleeding in the stomach and intestines as a common cause of iron deficiency anaemia. The black stool guide covers the other causes.

A made-up week on iron

Day	Stool	Notes
Mon	Type 4, darker than usual	Iron before breakfast, heartburn by mid-morning
Tue	Type 4, darker than usual	Took it after breakfast, no heartburn
Wed	Type 2, darker than usual	Strained, 4 glasses of water
Thu	Type 3, darker than usual	7 glasses of water, extra fruit, short walk
Fri	Black and tar-like	Dizzy, feel unwell

Monday and Tuesday follow the NHS how-to page: iron works best on an empty stomach, but you can take it with or after food if it upsets your stomach. With prescribed iron, check the change with your doctor or pharmacist. Wednesday’s constipation is on the NHS list. Friday is different. Black or tar-like poo is on the ferrous sulfate page’s straight-away list even without other symptoms, and MedlinePlus says to contact a provider right away if you feel dizzy or lightheaded. If that poo were also sticky and very smelly, the NHS stomach ulcer page says to call 999.

When to get help

• Talk to a doctor or call 111 straight away: poo that is black, tar-like or red, has blood in or on it, or you feel unwell in any other way (NHS ferrous sulfate page).
• Call 999 or go to A&E: poo with blood in it, or black, sticky and very smelly poo, vomiting bright red blood or vomit that looks like ground coffee, or severe tummy pain (NHS stomach ulcer page).
• Contact 111 now if you or a child takes more than the recommended dose. The NHS says an iron overdose in a young child can be fatal, so keep tablets out of reach.

Outside the UK, routes differ: MedlinePlus, a US government health site, lists black, tarry stools among reasons to call 911 or go to the nearest emergency room. Use your local emergency number for black, tarry stool and for the 999 signs. This article is general information, not medical advice.

How bellySignal helps

In bellySignal you log each stool’s Bristol type (1 to 7) and color, and flag blood or mucus if you see it. Water goes in as cups or mL, and urine color on the 1 to 8 hydration scale, so you can spot the dark pee the NHS mentions as a sign of dehydration. Notes and food triggers hold the tablet time and what you ate with it, a timeline to show your doctor or pharmacist. The app can’t tell iron-darkened stool from digested blood, and constipation has its own guide.

Your diary is stored on your device, and you can start logging without signing up. Optional login, health platform sync, and export are available. Some health values, including stool type, symptoms, and manually entered weight, are also sent to PostHog, which is enabled by default. The analytics switch in settings controls PostHog only. Firebase Analytics receives usage events without those health values and remains active. Privacy policy.