Three or four days into a course of iron, and you are planning your morning around the nearest loo. It is one of the more alarming ways iron can disagree with you, and one of the quietest reasons people give it up, often without telling a soul.
The fright usually lifts once you know what it is: the iron you do not absorb churning up the lower gut on its way out, and it is rarely a sign of harm. It is also less common than you might fear.
Of the three main ways iron upsets the gut, loose stools are the least common. In the largest analysis we have, 43 trials and 6,831 adults, they hit about 8 people in every 100 on ferrous sulfate, against 12 for constipation and 11 for nausea (Tolkien et al., 2015). The smallest group, and the most frightened.
I lose the odd client to this without ever hearing about it. They stop the tablet, the low iron sits there untouched, and months later they are back in my room more tired than before and none the wiser. It rarely needed to go that way.
Why do iron tablets cause diarrhoea?
Only a sliver of any iron tablet reaches your blood. A standard ferrous sulfate 200 mg tablet holds 65 mg of elemental iron, the part that counts, and a healthy gut takes up perhaps a tenth to a fifth of it, less again with food in the way (Pantopoulos, 2024). The rest travels on into the large bowel, day after day, and that surplus is where the trouble starts.
The leading explanation, and a sound one, is irritation. Unabsorbed ferrous iron is chemically reactive, and against the bowel lining it draws water into the gut and, in some people, hurries the whole works along.
What we do not have is the clinching study. No trial has measured gut transit on iron against a dummy tablet, so anyone who tells you exactly why your iron loosens you is going a step past the evidence.
What the dose also does is feed the wrong bacteria. Your colon is densely populated, and those microbes compete for iron much as you do (Paganini and Zimmermann, 2017). Pour in iron the body could not use, and the species that keep the lining settled, the bifidobacteria and lactobacilli, are crowded out by hungrier families such as E. coli, and the bowel wall grows measurably more inflamed.
There is a chemical edge to this, and it is the part that has made me wary of standard iron over the years. The loose iron reacts with the hydrogen peroxide your gut bacteria give off, through what chemists call the Fenton reaction, and throws off hydroxyl radicals, some of the most reactive molecules the body makes. It is corrosion, in effect, working on living tissue instead of metal.
We can measure it, too: two weeks of even a modest 19 mg daily dose roughly quintupled the loose, reactive iron in healthy volunteers' stools and lifted the free-radical activity there by about 40 per cent (Lund et al., 1999).
The broad effect is well counted. Across that same 6,831-adult analysis, ferrous sulfate roughly doubled gut side effects against a dummy pill (odds ratio 2.32; Tolkien et al., 2015). The microbial side shows starkest where the dose is large for the body: in one infant study, more than a quarter of babies on a high iron dose needed treating for diarrhoea, against fewer than one in ten given none (Jaeggi et al., 2015).
In grown women, twelve weeks of ferrous sulfate nudged up a virulence gene carried by E. coli, the fingerprint of a more troublesome strain gaining ground, even while overall diversity held steady (Finlayson-Trick et al., 2023). When a client says their iron goes through them like water, this is the machinery I picture.
Why does iron block one person and loosen the next?
Iron is far better known for bunging people up, which is what makes the diarrhoea so confusing. It does both. In the same review we keep coming back to, ferrous sulfate constipated about 12 people in every 100 and loosened about 8 (Tolkien et al., 2015). One salt, sending people in opposite directions.
Which way you go is settled more by your own gut than by the size of the dose. The same team looked for a dose-response and found none: bigger doses did not reliably bring more trouble. So the usual reflex, halve it and hope, only goes so far.
A smaller dose is still worth taking, since it leaves less unused iron to irritate the colon, but it will not reliably tip you from loose to settled. Your transit speed and the bacteria you already carry have more say in that than the milligrams do.
One thing here panics people for no reason. Iron turns stools very dark, almost black, and that is harmless and nothing like diarrhoea. Dark but formed is just spent iron on its way out. If that is the real worry, I have written separately on why iron blackens stools.
Is it the iron, or was your gut already unsettled?
Timing usually settles it. Diarrhoea that began within a few days of your first tablet, and eases on the days you forget to take one, is the iron almost every time.
The other way round deserves a careful look. If your bowels were unreliable before the box was opened, the iron may be a bystander rather than the cause. I have sat with people certain a supplement gave them months of loose stools, when in truth whatever was upsetting the bowel had also been quietly draining their iron, so the low reading and the diarrhoea simply arrived together. Several gut conditions behave exactly like that.
You will not untangle that at the kitchen table. The two markers we look at are a ferritin, which reads your iron stores, and a transferrin saturation, which shows how much of your iron-carrying protein is loaded. Take the whole picture to a nutritionist or qualified healthcare practitioner rather than experimenting on yourself.
How do you stop diarrhoea from iron tablets?
Most people settle this and keep taking iron. When someone has had enough and is ready to stop, here is what I try, easiest changes first.
- Put a little food with it. Iron on an empty stomach is at its most reactive going through, and a few mouthfuls of something blunt that and firm things up. You give up a little absorption for it, a trade I will take every time over a client who dreads the dose and quietly stops.
- Take less, or take it every other day. A smaller amount leaves less unused iron tipping into the colon. Alternate days do something neater: a 60 mg or larger dose trips a hormone called hepcidin that part-closes the gate on the next day's iron (Moretti et al., 2015), so spacing the doses absorbs more overall. On alternate days fractional absorption ran 21.8 per cent against 16.3 on consecutive ones (Stoffel et al., 2017).
- Drink more than you think you need. Diarrhoea takes water and salts out quietly, and a day or two of it adds up. Keep your fluids topped up while you adjust the dose, more so in warm weather.
- Change the form before you give up on iron itself. Ferrous sulfate is the harshest of the everyday salts. A gentler, lower-dose one such as ferrous bisglycinate leaves less behind to rile the colon. It is not a clean fix, though: that same bisglycinate has been shown to nudge those Enterobacteriaceae up, gentler symptoms or not.
- Know where the line is. Blood in the stool, diarrhoea that is severe or watery, anything still going after a week of the above, or the signs of dehydration, the light-headedness, the dark urine, the passing of very little, mean it is time to stop the iron and be seen. These are signs to act on, not to push through.
A label is not much help while you are choosing. "Gentle" carries no legal meaning and sits on plenty of ordinary ferrous salts. "Slow release" and "enteric coated" are at least honest about what they do, which is carry the iron past the stretch of gut that absorbs it, so less of it lands and fewer symptoms follow.
The largest tolerability review we have, 111 trials and over 10,000 patients, put modified-release versions top for comfort, with gut side effects under 4 per cent against about 30 on plain ferrous sulfate, and that comfort is paid for in absorption (Cancelo-Hidalgo et al., 2013). I have lost count of the clients who got nowhere on a slow-release tablet and were fine on a plain, well-absorbed one.
Which iron is gentlest if it keeps loosening you?
When two or three tablets have already beaten you, swapping to a fourth brand rarely helps, because the brand was never really the problem. What decides how your gut copes is the chemical form of the iron, and that is what has stayed the same through every switch you have tried. I will usually drop to a gentler, lower-dose tablet first, ferrous bisglycinate for choice, and a fair few settle there.
If that still will not sit, the next step is off tablets entirely, onto a liquid the gut barely has to break down. That is where Nano Iron comes in.
Nano Iron is not a ferrous salt at all. The iron is milled down to particles between 0.5 and 7 nanometres and carried in water, fine enough that a good part of a dose is taken up through the lining of the mouth and throat if you hold a capful there for a minute before you swallow. Because so much more goes in high up, far less travels on to the colon.
That leaves much less of the loose, reactive iron behind the chemistry above, and much less for the wrong bacteria to feed on. There is little trial data on liquid iron of this kind, so the case rests on how it behaves in the gut rather than on outcome studies.
A form that leaves so little reactive iron in the gut is, on that basis, far less likely than tablets to upset your bowels, and it tends to suit people who have been through two or three irons already and got nowhere.
It asks two things of you. It costs more than a strip of tablets, and the held mouthful feels odd for the first week before it becomes habit. For someone the tablet aisle has given up on, neither counts for much. The dosing and the customer reviews are on the Nano Iron page.
Whichever direction iron has taken you, my first move is the same: get more of it absorbed, so there is less of it rattling round the lower gut to stir up trouble of any kind. That is the whole point of how to increase iron absorption, and it is where I would start. If your trouble has been constipation or nausea rather than loose stools, those have pages of their own.
Frequently asked questions
Can iron tablets cause diarrhoea?
Yes, though less often than they constipate. It is the iron you do not absorb irritating the bowel and unsettling the bacteria in your colon, and ferrous sulfate is the usual culprit. For most people it settles once the dose, the timing or the form changes.
How do you stop diarrhoea when taking iron tablets?
Put a little food with the dose, drop to a smaller amount or every other day, and keep your fluids up. If that does not do it, change the form. Blood, severe or watery diarrhoea, or diarrhoea that will not let up means stop and get checked.
Can low iron itself cause diarrhoea?
Low iron on its own does not usually loosen the bowel. But some gut problems drain your iron and disturb your bowel at the same time, so if the diarrhoea came first, before the tablets, get tested rather than guessing.
Which iron is gentlest on the gut?
A lower-dose, well-absorbed iron leaves less behind to irritate. Ferrous bisglycinate is usually kinder than ferrous sulfate, and because a low-residue liquid such as Nano Iron is absorbed higher in the body, less iron reaches the colon and it is far less likely than tablets to upset your bowels.
Are the black stools from iron the same as diarrhoea?
No. Dark, near-black stools are a harmless and normal effect of iron. Diarrhoea is loose and frequent. Dark but formed is nothing to worry about.
How long does iron diarrhoea last?
Usually a few days to a fortnight as the gut settles, and sooner once you make the changes above. If it is severe, or still going after two weeks, stop pushing and have your iron levels checked.
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.
- 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
- Tolkien Z, Stecher L, Mander AP, Pereira DI, Powell JJ. Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis. PLoS One. 2015;10(2):e0117383.
- Pantopoulos K. Oral iron supplementation: new formulations, old questions. Haematologica. 2024;109(9):2790-2801.
- Paganini D, Zimmermann MB. The effects of iron fortification and supplementation on the gut microbiome and diarrhea in infants and children: a review. Am J Clin Nutr. 2017;106(Suppl 6):1688S-1693S.
- 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.
- Jaeggi T, Kortman GAM, Moretti D, et al. Iron fortification adversely affects the gut microbiome, increases pathogen abundance and induces intestinal inflammation in Kenyan infants. Gut. 2015;64(5):731-742.
- Finlayson-Trick E, Nearing JT, Fischer K, et al. The effect of oral iron supplementation on gut microbial composition: a secondary analysis of a double-blind, randomized controlled trial among Cambodian women of reproductive age. Microbiol Spectr. 2023;11(3):e05273-22.
- 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.
- Stoffel NU, Cercamondi CI, Brittenham G, 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: two open-label, randomised controlled trials. Lancet Haematol. 2017;4(11):e524-e533.
- Cancelo-Hidalgo MJ, Castelo-Branco C, Palacios S, et al. Tolerability of different oral iron supplements: a systematic review. Curr Med Res Opin. 2013;29(4):291-303.
