It is a well-known fact that iron supplements cause in addition to stomach pain and nausea (more seldom) constipation – the most likely side effect. Why does iron cause constipation?

Although iron bisglycinate is known for being stomach friendly among divalent iron salts and causes side effects more seldom [1], some people are still more gentle to iron than others. Therefore, iron bisglycinate can cause constipation for some people. The bigger the amount of iron consumed, the higher the risk for side effects [1]. For instance, side effects with Iron 30 mg and Iron syrup for children 25 mg are more rare, because the daily amount of iron is only 25–30 mg. Superiron is 60 mg iron, i.e. twice as much. However, Superiron is needed in case of diagnosed iron deficiency anemia and usually the choice for pregnant women. A lot of people do not have any side effects with iron bisglycinate independent on the amount consumed.

Iron has to change to ferrous iron Fe2+ to get absorbed by the intestine´s duodenum [1]. All OTC iron supplements sold in Estonia are in ferrous form. Ferric iron, i.e. Fe3+ (e.g. Ferrum Lek prescription drug) has to change first to ferrous iron Fe2+. Some amount of ferric iron will not get absorbed. Iron not absorbed by the body feeds bad bacteria in the colon. Also some amount of ferrous iron will not get absorbed.


  • prefer iron bisglycinate from OTC supplements (high bioavailability and stomach-friendly divalent iron, i.e. ferrous form)
  • when you start with Superiron 60 mg and know that previously iron supplements have caused constipation, make the pill into half and take only 30 mg during the first week
  • if no constipation appears during the first week, start using whole pill per day 60 mg
  • iron can also be taken every other day – then iron treatment takes more time, but the chance of getting side effects is smaller (during pregnancy it is advised to use iron daily due to limited time)


  • drink plenty of water – we often underestimate the importance of drinking water
  • exercise – sedentary lifestyle may cause constipation even without iron supplements
  • eat food rich in fibre: eat whole grain products, berries, fruit, vegetables, legumes -> fiber increases the amount of stool and promotes regular visits to the toilet
  • try dried black plums – a handful of dried plums per day also promotes healthy gut
  • visit the toilet regularly – avoid postponing your toilet visits
  • avoid food that promotes constipation: raw hard bananas, wheat products, a lot of blueberries and blackcurrants, menu rich in meat, a lot of cow milk and cheese
  • use fermented products: sauerkraut, yogurt (also plant-based yogurts are fermented products), pickles, natto, miso, tempeh, fermented soy products – fermented products contain live bacteria and enrich gut´s microflora

Some diseases and medication also promote constipation. Pay extra attention to preventing and relieving constipation if you:

  • are pregnant – metabolism slows down during pregnancy to ensure that the fetus gets all the necessary minerals and vitamins needed for normal growth. Read more about pregnancy and iron from HERE.
  • are elderly person -> elderly people suffer more from constipation even without iron treatments [2]
  • use certain medication: some examples include pain killers opioids, some Parkinson´s disease´s and psychiatric medication, diuretics (promote the amount of urine) and verapamil (e.g. used for heart disorders) [2]
  • are diagnosed with irritated bowel syndrome [2]

When you already suffer from bad constipation and visiting toilet is painful (rectum has a small wound and you may also see some light red blood), then postpone taking iron for the next couple of days. Consume more water, eat food rich in fibre and visit toilet regularly (wound usually heals already in a couple of days). When your stomach is back to normal, you can start taking iron again for example every three days. After a week you can already try taking iron every other day and if you feel okay after a week or two, you can also start using iron daily.

In case iron deficiency anemia has not developed, you can actually use iron supplements every other day (during pregnancy it is advised to take iron daily). Taking iron every other day prolongs iron treatment, but the cumulative absorption of iron is better and likelihood for side effects smaller [3]. In that case iron treatment lasts for 6 months instead of 3 and you can check iron related blood tests every 3 months (usually you are advised to check every 2 months). Taking iron every other day is also suggested for people with inflammatory diseases – in case of inflammatory diseases, the level of hepcidin is higher, which decreases the absorption of iron [3]. You can read about taking iron every other day and hepcidin from our blog.

In case of iron deficiency anemia, normalization of synthesis of hemoglobin is important and therefore, it is advised to take iron daily. When synthesis of hemoglobin has normalized, you can continue taking iron every other day to increase your iron stores (ferritin level).


[1] A.Aro, M.Mutanen, M.Uusitupa. Ravitsemustiede, 4.-7.köide, 2017.
[2] Ummetus. Terveyskirjasto Duodecim. 2020. Kustannus Oy Duodecim.
[3] N.Stoffel 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.The Lancet Haematology. Volume 4, issue 11, PE524-E533, November 01, 2017.

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