Iron Supplement Side Effects, and How to Take Iron So It Works

Iron supplements upset the gut: constipation, nausea, stomach pain, diarrhoea and black stools are reported by around a third of people on standard ferrous sulphate, and they are the main reason people stop before the deficiency is fixed.¹ Most of it is dose and timing rather than iron itself. Taking the same tablet on alternate days instead of daily absorbs more iron and causes fewer side effects, because a dose of iron shuts the gut's absorption gate for about a day, so the next day's tablet mostly passes through and irritates.^{2, 3} Knowing the elemental dose, the actual iron inside the compound, stops people taking far more than they need. And a gentler form, such as iron bisglycinate, is better tolerated at a matched dose for many.⁴ Black stools are harmless and expected. The two things to take seriously are iron taken without a test showing you need it, and iron within reach of children, for whom it is one of the commonest serious poisonings.

• Common and harmless: black or dark green stools, a metallic taste
• Common and fixable: constipation, nausea, stomach cramps, diarrhoea, heartburn
• Fixes: alternate days, lower elemental dose, with a small meal, a gentler form, more fluid and fibre
• Not for: anyone without a test showing deficiency; haemochromatosis; thalassaemia unless advised
• Keep away from children: iron overdose is a medical emergency

What each side effect means

Black stools. Unabsorbed iron darkens the stool, often to near-black or dark green within a day or two. It is harmless and expected and does not mean bleeding, though the two look alike, so mention it if you also have stomach pain or feel unwell. It stops when the iron does.

Constipation. The commonest complaint. Unabsorbed iron slows the bowel and hardens stool; more fluid, more fibre, movement and, if needed, a gentle laxative from the pharmacist all help, and alternate-day dosing reduces the unabsorbed share that causes it.

Nausea and stomach pain. Worst on an empty stomach and with higher doses. Taking iron with a small meal reduces nausea at the cost of some absorption, a trade worth making if the alternative is not taking it at all.

Diarrhoea and heartburn. Less common; both respond to a lower dose or a different form.

Metallic taste and stained teeth with liquid iron; drink it through a straw and rinse.

Skin. "Iron supplements side effects skin" is a rising search; iron does not cause rashes in most people, and a true allergic reaction to an oral iron is rare. Skin that improves on iron is deficiency being corrected. Any new rash with a new tablet deserves a pharmacist's look.

Know your elemental dose

The iron that counts is the elemental iron inside the compound, and labels lead with the compound. The common UK prescription tablets: 200 mg ferrous sulphate contains 65 mg of iron; 210 mg ferrous fumarate, 69 mg; 300 mg ferrous gluconate, 35 mg. Over-the-counter products range from 14 mg of iron, a dietary top-up, to liquid sachets at 5 to 14 mg, to high-dose tablets matching the prescription salts. A GP treating deficiency aims for about 65 mg of elemental iron on alternate days; taking that dose twice a day, as older advice suggested, roughly doubles the side effects for little extra benefit.^{2, 3} People taking a 14 mg "gentle" product for a confirmed deficiency, on the other hand, are under-treating and will wait a very long time for ferritin to move. Our hub on vitamins and minerals carries the units box for iron and the other nutrients where the label figure misleads.

Why alternate days work better

When iron enters the body, the liver releases hepcidin, a hormone that closes the gut's iron gate for roughly 24 hours. A tablet taken the next day meets a closed gate, so most of it stays in the gut, where it causes the side effects, and little is absorbed. Studies measuring absorption directly found that the same total iron given on alternate days was absorbed substantially better than daily, and a randomised trial in iron-depleted women confirmed that alternate-day dosing raised iron stores as well as daily dosing with fewer side effects.^{2, 3} It is now the usual advice. A single morning dose also beats a split dose, for the same reason.

Which iron supplements cause fewer side effects?

Any form at a lower unabsorbed dose, which is what alternate-day dosing achieves. Beyond that, iron bisglycinate, iron bound to the amino acid glycine, is absorbed well and in trials against ferrous sulphate and other salts produced fewer gastrointestinal side effects at a matched elemental dose.⁴ Ferrous gluconate carries less iron per tablet and is gentler for that reason. Liquid irons are gentle because the doses are small; they suit maintenance, not correction. Slow-release tablets release iron lower in the gut, which reduces nausea but also absorption, so they are not a first choice for deficiency. Vitamin C alongside helps absorption of all of them in single-meal studies, though a trial adding vitamin C tablets to iron found no extra benefit, so a glass of juice is sensible and a high-dose vitamin C tablet is not needed.⁵

How to take iron so it is tolerated

• Alternate days, one dose, in the morning
• An hour before food or two hours after if you can; with a small non-dairy meal if nausea wins
• Away from tea, coffee, milk, calcium supplements, antacids and levothyroxine by at least an hour, ideally two to four
• With orange juice or a vitamin C food rather than a high-dose tablet
• More fluid and fibre from day one, before constipation starts
• Switch form, not stop, if side effects persist; tell your GP or pharmacist
• Expect three to six months, and continue three months after the blood count is normal to rebuild stores

What will you notice when iron starts working?

Black stools within days. Energy and breathlessness improving within two to four weeks. Haemoglobin rising on a blood test at four to eight weeks, ferritin over months. Hair shedding slowing after two to three months and restless legs easing in that time if iron was the cause. If nothing has changed at eight weeks, the dose, the absorption or the diagnosis needs revisiting with your GP; some people need an infusion.

Who should not take iron

Anyone without a blood test showing they need it. Iron is one of the few nutrients the body cannot easily excrete, so taken when stores are full it accumulates, and the UK guidance level for people without deficiency is 17 mg a day. People with haemochromatosis, an inherited iron-overload condition affecting around one in 150 people of northern European descent, must not take iron. People with thalassaemia or sickle cell disease take it only on their specialist's advice. Men and post-menopausal women with low iron need the cause found, because gut bleeding is a possibility, before or alongside treatment. Iron and heavy periods explains the test and the UK threshold, and foods high in iron how to hold a healthy level afterwards.

Iron and children

Iron tablets are a leading cause of serious accidental poisoning in young children, because they look like sweets and a handful of adult tablets can be fatal to a toddler. Keep every iron product, including gummies and multivitamins with iron, locked away. If a child swallows any, call 999 or go to A&E immediately, with the packet; do not wait for symptoms.

What we don't know

• Whether bisglycinate is better absorbed as well as better tolerated. Some trials say so; the evidence is thinner than for tolerability.⁴
• The best dose for mild deficiency. Lower doses on alternate days may match higher ones; trials are ongoing.³

Frequently asked questions

What happens if you take iron supplements every day?

If you are deficient, daily works but absorbs less and causes more side effects than alternate days. If you are not deficient, iron accumulates and the UK guidance is 17 mg a day at most.^{2, 3}

Why does iron turn stool black?

Unabsorbed iron darkens it. It is harmless and expected; mention it only if you also have stomach pain or feel unwell.

Should you take iron at night or in the morning?

Morning, on alternate days, an hour before food if tolerated; absorption is best then and the single dose beats a split one.²

Which iron supplements don't cause constipation?

None is guaranteed, but iron bisglycinate at a matched dose, a lower dose on alternate days, and more fluid and fibre reduce it substantially.⁴

What not to do while taking iron supplements?

Do not take it with tea, coffee, milk, calcium or antacids; do not double the dose daily; do not take it without a test; and never leave it where a child can reach it.

Sources

1. Tolkien Z, et al. Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis. PLoS One, 2015. PubMed 25700159.
2. Stoffel NU, et al. Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women. Lancet Haematology, 2017. PubMed 29032957.
3. von Siebenthal HK, et al. Alternate day versus consecutive day oral iron supplementation in iron-depleted women: a randomized double-blind placebo-controlled study. EClinicalMedicine, 2023. PubMed 38021373.
4. Abbas AM, et al. Efficacy of ferrous bis-glycinate versus ferrous glycine sulfate in the treatment of iron deficiency anemia with pregnancy: a randomized double-blind clinical trial. Journal of Maternal-Fetal and Neonatal Medicine, 2019. PubMed 29843553.
5. Li N, et al. The efficacy and safety of vitamin C for iron supplementation in adult patients with iron deficiency anemia: a randomized clinical trial. JAMA Network Open, 2020. PubMed 33136134.
6. Snook J, et al. British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults. Gut, 2021. PubMed 34497146.

This article is general information, not medical advice. Take iron only after a blood test shows you need it, follow your GP's dose, and keep all iron products away from children.