Can Iron Supplements Cause Constipation? (Research-Backed)


Iron deficiency anemia is one of the most common nutrient deficiencies, especially for regular blood donors.

Iron supplements are the most affordable and convenient solution for most people, but unfortunately frequently cause a variety of side effects – including constipation for some people.

We’re going to quickly take a look at research on iron supplements and constipation to see how common this symptom is, and which types of products are less likely to cause constipation.

Is Constipation a Side Effect of Iron Supplementation?

The most common side effects of consuming iron supplements have been known for a long time. They mostly include (12):

Note that the severity of symptoms is dose-dependent (i.e. consuming more iron leads to more severe side effects).

Intravenous iron injections rarely give patients these side effects, but aren’t as convenient and can cost more depending on location.

Furthermore, some people are more at risk for side effects than others.

Research has shown that iron supplements are generally well tolerated in kids (3). However, iron supplements are associated with an approximately 3.5 times risk of constipation for pregnant women (4).

Elderly consumers also experience constipation more often while taking iron supplements, although they’re more likely to have constipation in general (5).


Constipation and related side effects like bloating and abdominal discomfort are well-established side effects of iron supplements.

Can You Prevent Constipation From Iron Supplements?

Supplement quality has come a long way in the last few decades to improve absorption and tolerability.

Manufacturers can use certain methods to reduce the amount of people who experience side effects from iron supplements by over 50% (6).

In addition, most past studies have looked at lower quality forms of iron, mainly:

  • Ferric iron
  • Ferrous iron (e.g. iron sulfate, or ferrous gluconate)

These are typically the cheapest iron supplements, but absorb the worst and are more likely to cause digestive issues like constipation.

But now, there are quite a few better options. The 2 most common “good” forms of iron in supplements are:

  • Chelated iron
  • Carbonyl iron

Research has shown that both of these absorb much better than the other forms of iron, and are significantly less likely to cause side effects (7,8).


Spending a bit more on iron supplements that are chelated iron or carbonyl iron can reduce the risk of developing side effects like constipation.

The Source of Iron Can Also Matter

One final aspect to consider is heme vs non-heme iron.

Heme iron is extracted from animal sources, while all plant-based iron supplements are non-heme iron.

This is important because one study found that side effects are significantly more common with products containing non-heme iron.

That’s likely due to the fact that non-heme iron is harder to absorb.

For those on a vegan diet, sticking to carbonyl or chelated iron will already do a lot to mitigate the risk of constipation. However, you’ll see that many of the best vegan iron supplements also contain vitamin C because that further boosts absorption of nonheme iron by a large amount.


Those on a restricted diet that need a plant-based iron supplement are more likely to experience side effects. However, picking a good form of iron that includes vitamin C can eliminate most or all of that extra risk.

What to Do If Iron Supplements Give You Constipation

While iron supplements can be very useful, they can also be dangerous.

Iron is not something your body can easily flush, and getting too much is quite easy and can lead to iron toxicity.

In addition, some research shows there may be consequences to long-term iron supplementation like chronic inflammation, which can raise the risk of many serious diseases (9).

Before taking iron supplements, it’s important to consult your doctor, and keep them updated if you do experience side effects like constipation.

About the author

Dale Cudmore

Your friendly neighborhood vegan from Toronto. Chemical engineer turned semi-professional soccer player and freelance nutrition writer. I've been vegan for years and try to make life easier for others by sharing what I've learned.