Nobody warns you about the second week. You start the iron, feel quietly virtuous, and then your stomach files a complaint: cramps, queasiness, a bathroom schedule that has stopped being yours. The NHS lists constipation or diarrhoea, tummy pain, heartburn and feeling sick as the usual suspects, plus darker-than-usual poo.[1] Its advice is oddly blunt: keep taking the tablets anyway.[1]
Easier said than done. So here's what genuinely makes a difference, and what's just folklore.
It isn't just you
If iron upsets your stomach, you're in good company. A 2015 systematic review in PLOS ONE pooled 43 randomised trials and 6,831 adults and found that ferrous sulfate significantly raised the odds of gastrointestinal side effects compared with placebo.[2]
One caveat: that review looked at ferrous sulfate, the standard prescription form, so it can't tell you how any other form compares. Take it as proof that iron upset is real and common, not as a character flaw.
Look at the dose first
The NHS says 17mg a day or less of iron from supplements is unlikely to cause harm, and it names constipation, nausea, vomiting and stomach pain as side effects of higher doses, over 20mg.[3] For context, adult women aged 19 to 49 need 14.8mg a day, while men and women over 50 need 8.7mg.[3]
So if a high-dose tablet is wrecking your week, it's worth a word with your pharmacist or GP. And if a GP prescribed it for a diagnosed deficiency, don't swap anything yourself; that's their call.
For everyone else who simply wants to top up, a lower daily dose is the obvious thing to try first. Oxford Origins' Vegan Iron Complex sits at 14mg of iron per capsule as ferrous fumarate, under that 17mg line, alongside 400mcg of folic acid and 100mcg of vitamin B12.
Food, timing and what's in your cup
This is where most of the easy wins live. The NHS says taking iron tablets with or soon after food may reduce side effects, and that a glass of orange juice afterwards may help your body absorb the iron.[1] It also flags large amounts of tea, coffee, milk and dairy as things that make iron harder to absorb, along with high-phytic-acid foods such as wholegrain cereals.[1]
Put those together and the classic breakfast of strong tea, porridge and a splash of milk is, annoyingly, the worst slot for your iron. A piece of toast, a glass of juice and a quick tea an hour or two later is kinder on both counts.
There's a trade-off, and it's worth being straight about it. Some people take iron on an empty stomach in the hope of absorbing more. If your stomach objects, food is the NHS-backed fix, and an iron tablet you can actually keep taking beats a theoretically better one you abandon on day nine.
Does every-other-day dosing help?
You may have seen this one. A small crossover study published in Haematologica gave iron-deficient, mildly anaemic women aged 18 to 45 ferrous sulfate tablets on consecutive days and on alternate days, using 100mg and 200mg doses, and found that absorption was greater with alternate-day dosing.[4]
Read that carefully, though. It measured absorption, not stomach comfort, and the doses were far above anything in a daily supplement. It's an interesting finding rather than a rule, and if you've been prescribed iron, the schedule is something to settle with your GP.
Get tested before you start
The least glamorous tip is the most useful. The NHS says iron tablets are recommended after a blood test shows low red blood cell levels, and that the cause should be identified first.[5] Taking iron you don't need is a reliable way to collect side effects with no upside. If you're not sure whether you're in that camp, our guide on who actually needs to supplement with iron is a good place to start.
Then keep a note for a week: what you took, what you ate with it, how your stomach felt. Patterns show up fast. For us, gold standard includes a dose you can stick to, and our Vegan Iron Complex is built around exactly that idea.
Frequently asked questions
Should I take iron on an empty stomach or with food?
The NHS suggests taking iron tablets with or soon after food if they upset your stomach, as this may reduce side effects. Some people prefer an empty stomach, but if you get nausea or cramps, food is the simplest thing to try first.
Why does iron make my poo dark?
Darker-than-usual poo is listed by the NHS as a recognised side effect of iron tablets. If you're worried, or it comes with other symptoms, ask your pharmacist or GP.
How much iron is safe to take every day?
The NHS says 17mg or less a day from supplements is unlikely to cause harm, and only to take more if a GP advises it. If you're unsure whether you need iron at all, read who actually needs to supplement with iron and speak to your GP.
Should I stop taking iron if it gives me side effects?
The NHS advice is to keep taking the tablets even if you get side effects, and to try taking them with or soon after food. If the side effects don't settle, talk to your pharmacist or GP rather than stopping on your own, particularly if the iron was prescribed.
Sources
- NHS — iron tablet side effects, taking them with food, orange juice, and tea/coffee/milk/wholegrain absorption advice. https://www.nhs.uk/conditions/iron-deficiency-anaemia/treatment/
- Tolkien Z et al., PLOS ONE 2015 — systematic review and meta-analysis of gastrointestinal side effects of ferrous sulfate (43 trials, 6,831 adults; OR 2.32 vs placebo). https://doi.org/10.1371/journal.pone.0117383
- NHS — iron recommended daily amounts, 17mg supplement guidance and side effects at higher doses. https://www.nhs.uk/conditions/vitamins-and-minerals/iron/
- Stoffel NU et al., Haematologica 2020 — iron absorption from supplements greater with alternate-day than consecutive-day dosing in iron-deficient anaemic women. https://haematologica.org/article/view/9379/68313
- NHS — iron deficiency anaemia: blood testing and identifying the cause before iron tablets. https://www.nhs.uk/conditions/iron-deficiency-anaemia/
At Oxford Origins, your wellbeing is our priority, and although we pride ourselves on our expertise in nutrition and supplementation, it's important to acknowledge the individuality of each person. Articles published by Oxford Origins are not intended to treat, diagnose, cure or prevent any disease, or replace the advice of your GP. We always recommend consulting with a healthcare provider if you encounter any health concerns, and we'll always be here to support you so you're never alone on your journey.
