Millions of people take over-the-counter calcium and vitamin D supplements, most often as protection against bone fractures. But these seemingly harmless pills may not be so harmless.

Calcium supplements have already been linked to a possible increased risk of heart attacks. And their value when it comes to preventing bone fractures has recently been questioned. Now there's another reason to not to use supplements to meet your daily required dose of calcium — with or without vitamin D — polyps in your colon.

Until more is known, people who have a history of pre-cancerous serrated polyps, especially women and people who smoke, should probably avoid calcium and vitamin D supplements.

Polyps are usually found in the lower part of the large intestine during a routine colonoscopy. They are small non-cancerous growths that are generally removed during a colonscopy because of the concern that they could eventually become cancerous.

Some research has suggested that calcium and vitamin D might prevent the formation of polyps, though the results of studies have been inconsistent. So researchers set out to dig deeper and try to determine if taking calcium and vitamin D supplements could reduce the risk of developing a particular type of polyps known as serrated polyps.

Over 2,000 people between the ages of 45 and 75 years with a history of polyps and who were scheduled to have a follow-up colonoscopy in three to five years were involved in the study. They were randomly divided into four groups. One group received daily supplements of calcium, another received only vitamin D, a third group were given both, and the remaining participants received no supplements until their next scheduled colonoscopy.

No immediate effect was seen on the number of serrated polyps among people taking either calcium or vitamin D, but six to ten years later, during a later phase of the study, researchers found a larger risk of serrated polyps among people taking only calcium supplements and those in the calcium and vitamin D together group. Women and smokers who took calcium supplements also had a higher risk of developing polyps. There was no effect found in the vitamin D only group.

More research is needed to confirm the results of this study, and the risks of supplementation with calcium and/or vitamin D have to be weighed against the benefits. Most people can get the calcium they need through their diet. Because calcium supplements are taken by millions of people, these findings could have significant implications for preventing colon cancer.

Until more is known, people who have a history of pre-cancerous serrated polyps, especially women and people who smoke, should probably avoid calcium and vitamin D supplements, or at least discuss the risks and benefits of supplementation with their doctors.

The study is published in Gut.