Learn About the Side Effects of Iron Supplements

Taking iron supplements may cause side effects such as nausea and constipation. Thus, it's important to stick to the dose recommended by your doctor.
Learn About the Side Effects of Iron Supplements
María Vijande

Reviewed and approved by the pharmacist María Vijande.

Written by María Vijande

Last update: 21 August, 2022

People use iron supplements in the prevention and treatment of iron deficiency anemia but their intake could lead to side effects when not taken properly. This mineral is helpful in situations where the body demands higher concentrations of it — during childhood and pregnancy.

Treatment of anemia doesn’t only encompass iron supplements. A person with this kind of deficiency must follow an iron-rich diet. Also, a blood transfusion may be necessary in more severe cases.

Types of iron supplements and their side effects

In its elemental form, iron is an unstable metal that easily oxidizes. It’s for this reason that most supplements come as complexes such as ferrous sulfate, ferrous fumarate, and ferrous gluconate. This is because these stabilize the iron.

All supplements contain the same type of iron though. What varies is how the body absorbs them or how these interact with food. In addition, some iron supplements may have more side effects than others — mainly at the gastrointestinal level.

A few capsules of iron supplements.
There are several types of iron supplements.

Ferrous sulfate is the best-known supplement and you must take it on an empty stomach as it could cause nausea and heartburn.

However, there are other iron supplements one can take with meals. The iron is formulated so as not to interact with food or other substances. Thus, it’s more bioavailable and has fewer side effects.

There are also iron supplements that contain other substances such as folic acid and vitamin B12. The doses are 325 mg for all three, although ferrous fumarate is more viable (33%).

How do these supplements work?

The human body can only absorb a certain amount of iron at a time. This is why professionals recommend taking them several times a day, instead of taking just one dose. It promotes the assimilation of iron.

In addition, it’s best to take iron on an empty stomach so as to ease assimilation so take it one hour before you eat. It’s important for iron not to interact with other medications; therefore, take it at least two hours before or after taking another drug. Especially if these are supplements with calcium, zinc, or folic acid.

A woman taking a pill.
Keep in mind that iron supplements can interact with other medications. Therefore, you should take them in advance.

For greater absorption, take the iron along with a glass of orange or tomato juice, as these contain vitamin C and help with its assimilation. There are several causes of anemia, although they’re usually due to sustained iron deficiency.

The widespread practice for the prevention or treatment of anemia in women is daily iron supplementation. Furthermore, it’s sometimes combined with folic acid, other vitamins, and minerals for three months.

However, there’s a link between daily iron supplementation and side effects such as nausea or constipation. Thus, its intake once, twice, or three times a week administration is an effective and safer option than daily doses.

Possible side effects of iron supplements

The side effects vary, depending on the type of iron complex used. The most common are:

  • Acid reflux and heartburn
  • Nausea and vomiting
  • A metallic taste in the mouth
  • A feeling of fullness
  • Dark stools
  • Diarrhea or constipation

In addition, nausea and gastric discomfort may increase with the dose of iron complexes. These usually appear about 30 to 60 minutes after taking them. Nevertheless, they’ll probably disappear after the first three days of treatment.

Adopt healthy habits to reduce constipation, typical of this medication, such as increasing the consumption of fibers from fruit and vegetables, and being physically active.

Take iron complexes with meals whenever possible. Some supplements may also contain folic acid and vitamin B12, which also help against anemia.


You must take iron supplements only upon medical advice. This deficiency stabilizes within two months but it takes the body between six months and a year to recover from it.

All cited sources were thoroughly reviewed by our team to ensure their quality, reliability, currency, and validity. The bibliography of this article was considered reliable and of academic or scientific accuracy.

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  • 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. Published 2015 Feb 20. doi:10.1371/journal.pone.0117383
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  • OMS. (2014). Administración diaria de suplementos de hierro y ácido fólico en el embarazo. Organización Mundial de La Salud. https://doi.org/10.5923/j.instrument.20170601.01

This text is provided for informational purposes only and does not replace consultation with a professional. If in doubt, consult your specialist.