If you have started taking iron, felt dreadful within a week and stopped, you are in very large company. It is one of the most commonly abandoned supplements there is, and the reason is more interesting than most people are told.
Your body limits how much iron it lets in
Iron is one of the few nutrients your body cannot excrete in any meaningful quantity. So rather than getting rid of excess, it controls how much gets in.
The gatekeeper is a hormone called hepcidin. When you take a dose of iron, hepcidin rises, and while it is elevated your gut absorbs less. It stays raised for roughly 24 hours.
Which creates a problem with daily dosing. Tomorrow morning's tablet arrives while your body is still limiting absorption from today's. You take on a smaller share of it, and the iron you do not absorb continues through your digestive tract.
That unabsorbed iron is a large part of the discomfort. It irritates the gut lining and it changes the bacterial environment in the colon, which is where the nausea, the constipation and the general awfulness come from.
Why the doses are so large
The numbers on an iron label are easy to misread, and the gap between them explains a lot.
The 18mg a day recommended for women aged 19 to 50 is an intake figure. It already assumes you will absorb only a fraction of what you eat, around 18% from a normal mixed diet. So what your body actually needs to take up is closer to 1.5 to 2mg a day. After menopause it is about 1mg.
Correcting a deficiency is a different job again. You are not topping up a daily requirement, you are refilling stores that have run down, and that means pushing a great deal more in to get a little more out. Which is why therapeutic doses start at 60mg and climb from there.
And that is precisely the amount that provokes a strong hepcidin response. The dose is large because absorption is inefficient, and the size of the dose then makes absorption less efficient still. That circle is the whole problem, and it is why the timing turns out to matter.
It is also why iron from food does not behave this way. Spread across meals in ordinary amounts, it never trips the response the way a tablet does.
What happens if you take it every second day instead
Researchers have tested exactly this, and the results have changed how a lot of clinicians think about iron.
One trial followed 150 women for six months on the same total amount of iron, some taking it daily and some every second day. By the end, 3% of the alternate day group were still iron deficient, against 11% of the daily group.
Same iron. Better result. Taken half as often.
The same research also overturned older advice about splitting a dose across the day. Taking iron twice daily pushes hepcidin higher still and leaves you absorbing less, so a single dose is better than two.
The form question
There is a second variable most people are never told about.
Iron sulfate is the cheapest form and the one you will most often be handed. It is also the hardest on the stomach.
Iron bound to an amino acid, usually sold as ferrous bisglycinate, is better absorbed and better tolerated. In a pregnancy trial, 25mg of bisglycinate was as effective as 50mg of sulfate, with significantly fewer gastrointestinal complaints.
So if iron has made you miserable, there are two levers, and most people are told about neither.
Why prenatals get abandoned
This is, in my experience, one of the main reasons women stop taking their prenatal.
Some prenatals carry a high dose of iron. The requirement in pregnancy is 27mg a day. So a woman with no identified deficiency can end up taking a therapeutic dose she was never assessed as needing, feel awful, and conclude that prenatals do not agree with her.
Nobody prescribed it. She chose a bottle off a shelf, and the dose inside it was decided by a manufacturer rather than by anyone who looked at her.
If that is you, please do not simply stop. The folate and the iodine alongside the iron do need to be taken daily, and preconception and early pregnancy is precisely the window where they matter most. It is a conversation about the dose and the form, not a reason to give up on the whole thing. A pharmacist, GP or your pregnancy care team should be able to discuss options with you.
Before you change anything
Two different situations, and they need different handling.
If a doctor has put you on iron for a diagnosed deficiency, that dose was chosen deliberately, often to correct a specific shortfall within a timeframe. Take the frequency and form question to them rather than adjusting it yourself.
If you picked something up off a shelf, nobody assessed you at all. That is worth a conversation of its own, and the question is simply whether what is in it suits you.
Either way, do not start iron without knowing your levels. Iron is not a supplement to take on the assumption that more is better. People with haemochromatosis and other iron storage conditions absorb and retain more than they should, and additional iron does real harm. This is the one nutrient where taking it just in case is a genuinely bad idea.
Where Greenher sits in this
Since we sell a product containing iron, it would be strange to write all of the above and say nothing about it.
Greenher Daily contains 12mg of iron per serve, as ferrous glycinate.
Against the absorbed requirement described earlier, 1.5 to 2mg a day for menstruating women, that lands as a meaningful daily contribution rather than the third of a requirement the intake figure implies. It is a nutritional amount, nowhere near the 60mg territory where the hepcidin problem starts. The Upper Level of Intake, for reference, is 45mg.
If you are over 51
The intake recommendation drops to 8mg a day once periods stop, so 12mg sits above it.
In practice this matters less than it sounds, because of the same hormone we have been talking about. When your stores are comfortable, hepcidin stays high and you absorb only a small fraction of what you eat. When they are low, absorption climbs. Your body is doing the sorting, and it is generally good at it.
If you are menstruating and actually low
This is the more important one.
If a blood test has shown you are deficient, 12mg in a daily food is not going to fix it. Refilling depleted stores takes a therapeutic dose, prescribed and monitored, and that is a conversation with your GP. Greenher can sit alongside that. It cannot do that job, and I would rather say so than have anyone wait on it.
If you have haemochromatosis
Or any other condition that causes you to store excess iron, the self-regulation described above is exactly what does not work properly. A daily product containing iron is something to check with your doctor before taking. That applies to ours as much as anything else.
Every nutrient in the formula is listed with the amount and the form on our formula page, with percentages calculated against women's requirements rather than an average adult.
The takeaway: Iron raises hepcidin for around 24 hours, so a daily dose is partly blocked and the unabsorbed portion causes most of the discomfort. Over six months, alternate day dosing left fewer women deficient on the same total iron. Bisglycinate is better tolerated than sulfate. Take both questions to your doctor rather than acting alone, and never start iron without knowing your levels.
The science behind this post:
- Stoffel NU, Cercamondi CI, Brittenham G, et al. (2017). 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 Haematology. Alternate day dosing increased the proportion of iron absorbed compared with daily dosing, and twice daily split dosing raised hepcidin further and reduced absorption.
- von Siebenthal HK, Moretti D, Zimmermann MB, Stoffel NU (2023). Alternate day versus consecutive day oral iron supplementation in iron-depleted women: a randomised double-blind, placebo-controlled study. eClinicalMedicine. 150 women given the same total amount of iron over six months. Iron deficiency remained in 3% of the alternate day group and 11.4% of the consecutive day group.
- Stoffel NU, Zeder C, Brittenham GM, Moretti D, Zimmermann MB (2020). Iron absorption from supplements is greater with alternate day than with consecutive day dosing in iron-deficient anemic women. Haematologica, 105(5). The proportion of iron absorbed was 40 to 50% higher with alternate day dosing.
- Moretti D, Goede JS, Zeder C, et al. (2015). Oral iron supplements increase hepcidin and decrease iron absorption from daily or twice-daily doses in iron-depleted young women. Blood, 126(17), 1981-1989.
- Milman N, Jonsson L, Dyre P, Pedersen PL, Larsen LG (2014). Ferrous bisglycinate 25mg iron is as effective as ferrous sulfate 50mg iron in the prevention of iron deficiency in pregnancy. Journal of Perinatal Medicine. Gastrointestinal complaints significantly less frequent with bisglycinate.
- National Health and Medical Research Council. Nutrient Reference Values for Australia and New Zealand: Iron. RDI 18mg for women 19 to 50, 8mg from 51, 27mg in pregnancy. Upper Level of Intake 45mg. nrv.gov.au