You glance into the loo a few days after starting iron, and the colour stops you cold. Near-black, darker than anything you have produced before. It is one of the most common reasons people contact a clinic in a quiet panic, and one of the most reliably harmless.
For the great majority on iron, a dark or near-black stool is simply spent iron on its way out, and nothing is wrong. There is one thing worth checking, and I will come to it: whether the stool is dark but ordinary, or black and tarry, which is a different matter. The first is the iron. The second is worth acting on.
I reassure someone about this most weeks. The colour looks alarming precisely because it is so dark, yet the darkness on its own is not the warning sign people take it for. What the stool looks and feels like matters far more than the shade.
Why does iron turn your stools black?
Only a small share of any iron tablet is absorbed. A standard ferrous sulfate 200 mg tablet carries 65 mg of elemental iron, and a healthy gut takes up perhaps a tenth to a fifth of it (Pantopoulos, 2024). The rest, the great majority, travels on into the lower bowel.
To put a number on it, of the 65 mg in a tablet we take up only around 7 to 13 mg; the remaining 50-odd milligrams is what carries on to the colon, day after day (Moretti et al., 2015). That surplus is the raw material for the colour.
The reason so much goes unused is the way iron is taken in. We absorb it high up, in the duodenum just past the stomach, and only so much can cross at once. A large single dose overwhelms that limited gate, and the excess simply carries on through.
Down there, the iron we do not absorb meets the hydrogen sulfide your gut bacteria give off, and the two react to form iron sulfide, a black, insoluble compound. That is the colour you are seeing. It is a metal meeting sulphur and turning dark, the same sort of reaction that tarnishes silver, and it is harmless here.
So the darkening is really a read-out of the iron you did not absorb. The more unused iron reaching the colon, the darker things tend to look. It tells you the tablet has passed through you, not that anything has gone wrong inside.
Do all iron supplements turn stools black?
More or less, yes, because every oral iron leaves some unabsorbed, and that surplus is what darkens the stool. What changes between them is how much. The harsher and higher the dose, the more iron reaches the colon, and the darker things tend to go.
A high-strength ferrous sulfate tablet usually blackens stools the most, since it leaves the largest unabsorbed load behind. A lower-dose form such as ferrous bisglycinate, or a liquid taken up high in the body, tends to leave less and darken less. The colour roughly tracks the residue, so it is not a mark of one brand being better than another, only of how much iron passed through unused.
That includes iron you might not think of as a supplement. A multivitamin with a decent slug of iron, or a separate iron tonic, darkens stools the same way a dedicated tablet does, so it is worth tallying everything you take rather than only the obvious one.
How soon does it start, and what does it look like?
The colour usually turns within a day or two of your first dose, occasionally the same day. It can run anywhere from a dark brown to a deep, near-black, and occasionally a dark green. None of those shades is a worry in itself.
Texture is the part that matters. Iron darkening looks matte and stays formed, much like an ordinary stool that has simply gone dark. It does not turn sticky, and it does not take on a strong smell. Once you stop the iron, the colour returns to normal within a few days as the last of the unabsorbed iron clears.
It is normal for the exact shade to vary from day to day, and from one dose to the next. What you have eaten, how much you drank and how quickly things moved all nudge the colour, so do not read too much into a darker day or a lighter one. The texture staying ordinary is the reassurance, not a constant shade.
If you are unsure, a quick look is usually enough to settle it. A dark but solid, ordinary-looking stool is the iron almost every time, and most of the worried clients I see relax the moment they describe the texture rather than the colour.
Why is my stool dark green instead of black on iron?
Green rather than black is common and just as harmless. It usually means the iron, and the bile your gut uses to digest, are moving through you faster than usual, so the colour has not had time to darken fully. A looser or quicker stool on iron often comes out more green than black for that reason.
Green can also come from what you eat, a run of dark leafy vegetables in particular, so it is not always the iron at all. Either way, a green or greenish-black stool that is otherwise normal is not a worry. The pattern to watch stays the same whatever the shade: tarry, sticky and foul-smelling is the one to act on.
When is a black stool a red flag, not the iron?
There is one pattern that is not the iron, and it is the one to learn. A stool that is jet-black and genuinely tarry, sticky like tar, with a strong and offensive smell, can be a sign of bleeding higher up in the gut, where blood is partly digested on its way through and turns black. That is a different thing from the dry, dark, formed stool iron produces.
The reason a bleed shows up black rather than red is that blood from higher up, in the stomach or upper bowel, is partly digested on its long way through, which darkens it and gives it that tar-like quality. Bleeding lower down, nearer the exit, tends to look red instead. The black, tarry kind is the one people most often mistake for their iron.
Take it seriously if a black stool is tarry and foul-smelling, or arrives with light-headedness, weakness, stomach pain, breathlessness, or any vomiting of blood or of what looks like coffee grounds. That combination needs urgent medical attention rather than wait-and-see: contact a doctor urgently or call NHS 111.
It is uncommon, but it is the one situation where the colour is telling you something real. In clinic we treat that pattern as a possible bleed until it is ruled out, and the same caution serves you well at home.
What else can turn your stools black?
Iron is the commonest harmless cause, though it is not the only one, and a couple of others catch people out.
On the food side, black pudding, a large helping of blueberries or blackberries, dark liquorice and the odd very dark meal can all deepen the colour. Among medicines, bismuth indigestion remedies such as Pepto-Bismol are the classic culprit, blackening stools within a day, and activated charcoal does the same. None of these is a problem.
The thread running through all of them is the one we have already drawn: a black stool that is dark, formed and ordinary in texture is almost always a harmless stain, whether from iron, food or a medicine.
What should you do if your stool turns black on iron?
For the ordinary dark stool, the answer is reassuringly little. If it is dark but formed, with a normal texture and smell, and you are otherwise well, there is nothing to do. It is expected, it is harmless, and it clears within a few days of stopping the iron whenever you do.
It helps to run the same quick check we do. Did the colour start within a day or two of the iron? Is the stool dark but solid, rather than tarry and sticky? Do you feel well in yourself, with no dizziness, no stomach pain and no blood? If the answers are yes, the iron is almost certainly the cause, and you can carry on.
You do not need to stop the iron, change brand, or take it differently for the colour alone. If the darkness genuinely bothers you, a lower dose or a better-absorbed form will lighten it, but that is a preference rather than a fix for a problem. The one time to act is when a stool turns tarry and foul-smelling, or comes with feeling faint or unwell, when you should stop and seek urgent medical advice.
Does the black colour clear up on its own?
While you keep taking iron, expect the dark colour to stay. It is a steady feature of being on iron rather than a phase that passes, and it does not build up or worsen, so there is nothing harmful to clear.
Once you stop, or finish a course, the colour returns to normal within a few days, as the last of the unabsorbed iron works its way through. If you have stopped iron altogether and stools stay black beyond a week or so, that is worth raising with a nutritionist or qualified healthcare practitioner, since by then the iron is no longer the likely cause.
Is a black stool a sign your iron is working?
It is a fair question, and the answer is partly yes, partly no. The colour does tell you the iron is going down and passing through you, and that some of it is the unabsorbed surplus on its way out. What it cannot tell you is whether your iron stores are actually climbing.
That is a separate measurement. The markers we look at are ferritin, which reflects your iron stores, and a transferrin saturation, the share of your iron-carrying protein that is loaded with iron. A dark stool says nothing about either. Someone can have textbook-black stools and still be under-dosed, or absorbing poorly.
So the colour only confirms the tablet is going down. Whether your iron is actually recovering is a separate question, and the only way to answer it is to test, before you start and again after a couple of months, then read those numbers with a nutritionist or qualified healthcare practitioner.
Does a gentler, low-residue iron darken stools less?
Since the black comes from the iron you do not absorb, an iron that leaves less behind gives the colour less to work with. The logic holds up: less unused iron in the colon means less iron sulfide forming, and less darkening. A smaller or better-absorbed dose tends to blacken stools less than a large ferrous one.
I would not chase that as a reason on its own, mind. The darkening is harmless either way, so a stool that keeps its usual colour is a convenience rather than a health gain.
It does connect to the form of the iron, though. Nano Iron is not a ferrous salt. The iron is milled to particles between 0.5 and 7 nanometres and suspended in water, fine enough that a good part of a capful is absorbed in the mouth and throat, before it is even swallowed, if you hold it there for a minute.
Because far more goes in early, far less reaches the colon to form iron sulfide, so it tends to darken stools less than a tablet does. Some darkening can still happen with any iron, so I would not promise a spotless result, and the case rests on the mechanism rather than on trials that counted stools.
The more useful thing that low residue buys you is less of the reactive, unabsorbed iron that troubles the gut in the first place (Lund et al., 1999), which is why it is far less likely than tablets to upset your bowels.
It costs more than a strip of tablets, so the real question is what you would be paying for. If the colour is your only complaint, that is a poor reason to switch, because the stain was never going to harm you. If standard iron has also been turning your stomach or blocking you up, the sums look different. The dosing and the customer reviews are on the Nano Iron page.
Dark stools are the most visible of iron's effects and the least worth losing sleep over. The ones that actually drive people off their iron are nausea, constipation and loose stools, and each comes down to how much unused iron the gut is left holding. If one of those is your real trouble, start with absorbing more from a smaller dose.
Frequently asked questions
Is a black stool from iron dangerous?
No. A dark or near-black stool that stays formed and ordinary in texture is harmless spent iron, the unabsorbed part of your dose on its way out. The only black stool to act on is one that is tarry and foul-smelling, or comes with dizziness, weakness or stomach pain.
Does a black stool mean I am taking too much iron?
No. It reflects how much iron you do not absorb, not a dangerous level in the body. Even a normal dose leaves most of its iron unabsorbed, and that is what darkens the stool. If you are unsure of your dose, have your levels checked rather than guessing.
Does a black stool mean the iron is being wasted?
Not wasted as such. Even iron you absorb well leaves an unabsorbed share behind, and that share is what colours the stool. A dark stool is a normal part of taking iron, not a sign the dose is going to waste.
Can low iron itself cause black stools?
No. Low iron does not darken stools; the colour comes from the iron in a supplement, not from a deficiency. If your stools are black and you are not taking iron or any of the foods and medicines above, that is worth getting checked rather than putting down to low iron.
Does taking iron with food stop the black colour?
No. Food changes how much you absorb a little, but the colour comes from the unabsorbed iron either way, so the stool will still darken. Take iron with a little food if it settles your stomach, but do not expect it to change the colour.
Will a multivitamin with iron turn my stools black?
It can, if it contains enough iron. The colour comes from the iron itself, not from whether it is a dedicated supplement, so a multivitamin, a tonic or a tablet can each do it. Add up all your sources if you are unsure what is responsible.
Does iron given by injection or drip cause black stools?
Usually not. Iron given into a vein bypasses the gut, so there is no unabsorbed iron passing through the bowel to darken the stool. Black stools are a feature of oral iron, the kind you swallow, rather than iron given by infusion.
My baby or child has dark stools on iron. Is that normal?
Yes, the same harmless darkening happens at any age. A tarry, sticky stool, or one with blood, or a child who seems unwell with it, is the exception and needs prompt medical advice. Keep iron supplements out of reach, as too much iron is dangerous for young children.
How can I tell if it is the iron or internal bleeding?
Texture and smell. Iron is dark but matte and formed, with no unusual odour. A bleed tends to be jet-black, tarry and sticky, with a strong offensive smell, often alongside feeling faint or unwell. If in doubt, treat it as the second and seek urgent medical advice.
Will switching to a low-residue iron stop it straight away?
Not necessarily, and not completely. A low-residue iron such as Nano Iron leaves less behind to form the black compound, so it tends to darken stools less, but any iron can still tint the stool. The colour change is harmless whichever iron you take.
A note on safety
- Keep out of the sight and reach of children. Taking too much iron can be harmful, and accidental overdose is a serious risk in young children.
- Do not exceed the recommended daily amount.
- A black, tarry, foul-smelling stool, or one with blood, or black stools alongside dizziness, weakness or stomach pain, needs urgent medical attention. Contact a doctor urgently or call NHS 111.
- If you are pregnant, breastfeeding, taking medication or managing a health condition, speak to a nutritionist or qualified healthcare practitioner before starting iron.
- Have your iron levels checked before and during supplementation, so you know how much you actually need.
- Food supplement. Not intended to diagnose, treat, cure or prevent any disease. A food supplement should not replace a varied, balanced diet and a healthy lifestyle.
Written by Cara Hayes, MSc Clinical Nutrition.
References
- Pantopoulos K. Oral iron supplementation: new formulations, old questions. Haematologica. 2024;109(9):2790-2801.
- Moretti D, Goede JS, Zeder C, et al. Oral iron supplements increase hepcidin and decrease iron absorption from daily or twice-daily doses in iron-depleted young women. Blood. 2015;126(17):1981-1989.
- Lund EK, Wharf SG, Fairweather-Tait SJ, Johnson IT. Oral ferrous sulfate supplements increase the free radical-generating capacity of feces from healthy volunteers. Am J Clin Nutr. 1999;69(2):250-255.
- National Health Service. Black or bloody poo and bleeding from the bottom. NHS; 2023.
- Cleveland Clinic. Melena (black, tarry stools): causes and when to seek care. Cleveland Clinic; 2023.
