Oral iron supplements may cause or worsen constipation, by making stools harder. A doctor can advise about supplementary iron and any gastrointestinal side effects.

Constipation affects around 16 in every 100 adults and 33 in every 100 individuals over 60 years. People with constipation may have hard stools that are difficult to pass and fewer than three bowel movements a week. They may also feel that they have not completely emptied their bowels.

Some medications and dietary supplements can worsen constipation. Individuals who take iron supplements for iron deficiency anemia or to obtain iron that they do not get in their diet may be at risk for constipation.

This article explores whether iron supplements cause constipation, examining the scientific evidence. It also offers tips for managing constipation and discusses when to contact a doctor.

Further resources

For more in-depth resources about vitamins, minerals, and supplements, visit our dedicated hub.

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The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) advises that iron supplements may worsen constipation.

According to a 2021 review, oral iron is the most common treatment for iron deficiency anemia due to its low cost, high bioavailability, and effectiveness.

However, the review authors noted that iron supplements may cause gastrointestinal side effects, such as constipation, abdominal pain, and bloating, in up to 60% of people who take them. The type of oral iron that doctors prescribe most is ferrous sulfate, but this has a high frequency of side effects compared with ferric iron sources.

The review further explains that iron supplements typically contain more iron than the body can absorb. Therefore, a large amount remains in the gut, affecting the balance of gut bacteria. In addition, older research suggests that oral iron may generate free radicals and cause inflammation.

A 2020 review notes that the mechanisms by which iron causes constipation are unclear. However, scientists think that excess iron ions in the stomach cause more water transportation into the intestines. This water is then distributed from the lower part of the gastrointestinal system to other areas of the system to maintain the acid-base balance.

This may result in stool hardening and constipation as water helps soften stools and make them easier to pass.

A person may be able to manage constipation by:

  • eating more fiber by including foods such as vegetables, whole grains, legumes, fruits, and nuts in the diet
  • drinking plenty of water
  • avoiding processed foods, fast food, and foods with little or no fiber, such as meat
  • staying physically active
  • trying bowel training by aiming to have a bowel movement at the same time each day — for example, 15-45 minutes after eating breakfast
  • asking a doctor which supplements or medications worsen constipation and if it may be safe to stop taking them
  • asking a healthcare professional about whether taking laxatives may be suitable

People with constipation that does not resolve should speak with a doctor. A healthcare professional may recommend over-the-counter medications such as:

In addition, a doctor may prescribe the following medications:

Depending on an individual’s health history, doctors may also recommend biofeedback therapy or surgery.

People should also consult a doctor to determine if they have any underlying medical conditions related to constipation or iron deficiency. Additionally, if someone with constipation has any of the following symptoms, they should seek medical attention as soon as possible:

Iron supplement safety

High doses of iron can be harmful and interact with medications. The daily upper limit of iron from all sources, which experts do not recommend exceeding, is 45 milligrams for adults.

However, some people, such as those with hemochromatosis, should not take iron supplements. Therefore, people should consult a doctor before supplementing iron.

Some pregnant or nursing people take iron supplements under a doctor’s guidance. However, if they become constipated, they should seek further advice.

People may take iron to help treat iron deficiency anemia or to supplement their diet. However, oral iron supplements may cause or worsen constipation. Experts suggest that unabsorbed iron in the gut may cause stools to harden and other gastrointestinal side effects, such as pain and bloating.

A person should consult a doctor before supplementing iron or if they think they have constipation. Eating fiber, staying hydrated, and being physically active may help avoid constipation. A doctor may also recommend medications to soften stools or make bowel movements easier.