How to Choose the Right Calcium to Reduce Constipation

It’s well known that iron supplements, especially in higher doses, can lead to constipation. A reader recently asked if the same concern applies to calcium supplements. While less commonly discussed, calcium may also slow bowel movements in some people, particularly when taken in larger amounts.

Some types of calcium supplements are more likely to cause constipation

Calcium is well recognized for supporting bone health, but it also helps regulate muscle contraction and nerve communication. Taking too much calcium may slow intestinal muscle activity (peristalsis), which can contribute to constipation. Not everyone notices this effect, but studies suggest that some forms of calcium are more likely to cause digestive issues. For instance, a 2018 study¹ reported that calcium carbonate was more often associated with constipation and bloating compared with other types of calcium.

Three strategies to help manage constipation from calcium supplements

1. Switch to Calcium Citrate

Research shows that calcium citrate is gentler² on the digestive system and is not associated with constipation in the same way calcium carbonate is. When choosing a supplement, check the ingredient list carefully because the front of the package does not always specify the form of calcium.

2. Choose a Calcium-Magnesium Combination Supplement

If you have a sensitive digestive system, consider a calcium supplement that also contains magnesium. Magnesium can help counteract constipation since some forms of magnesium have a natural laxative effect. However, not every magnesium supplement works this way.

Two forms in particular have been shown to be effective:

Magnesium citrate: This draws water into the intestines, softening stool and making it easier to pass.

Magnesium oxide: This has a similar effect by increasing water in the intestines. A small 2019 study³ from Japan tested magnesium oxide in 34 women with mild to moderate constipation. More than 70 percent of participants reported overall improvement in their bowel symptoms.

When choosing a calcium-magnesium supplement, check both the source of the calcium and the type of magnesium. For example, a product that combines calcium citrate with magnesium citrate and magnesium oxide may be less likely to cause constipation.

3. Take Smaller Doses, Split Into Two Servings

Some calcium supplements provide very high doses in a single pill, often 500 to 600 milligrams. Taking a large dose all at once can increase the risk of constipation. If possible, choose a lower-dose supplement, such as 250 to 300 milligrams, and take it twice a day (for example, morning and evening). Splitting the dose may reduce digestive side effects while still meeting your calcium needs.