Ask the Doctors - Do antibiotics increase the risk of colon polyps?

Dr. Robert Ashley, MD

Dear Doctor: A new study said that prolonged antibiotic use is tied to colon polyps. But “prolonged” meant “two weeks or more.” I had a couple of bacterial infections in a six-month timespan. How worried should I be?

Antibiotics have undoubtedly revolutionized medicine, saving countless lives against multiple types of bacterial infections. However, with antibiotics readily available and a society that craves a quick fix, antibiotics have been overused. They are often prescribed needlessly for upper respiratory symptoms that are not due to bacterial causes, for example. Such overuse has led to the formation of antibiotic resistance; alterations of normal bacterial populations in the intestinal, oral and nasal cavities; and unnecessary side effects from the antibiotics themselves. As for whether antibiotics can increase the risk of precancerous polyps in the colon, let’s take a look at the evidence.

The study to which you’re referring, published in the Journal Gut, reviewed the antibiotic usage of 16,642 female nurses age 60 or older. In 2004, the women filled out a questionnaire reporting the amount of antibiotics they used between the ages of 20 and 39 and between the ages of 40-59. In 2008, the women filled out another questionnaire reporting their antibiotic usage between 2004 and 2008. All of the women had at least one colonoscopy between 2004 and 2010.

Researchers found that women who took antibiotics between the ages of 20 and 39 had an increased risk of colon polyps compared to women who hadn’t taken antibiotics. The increased risk was relatively small for women who had taken antibiotics only for 1 to 14 days within that 20-year period, but the risk increased significantly, by about 1.4 times the risk of nonusers, among women who took antibiotics for 14 days to 2 months. That heightened risk didn’t increase further among women who took antibiotics for greater than 2 months.

For women who took antibiotics between ages 40-59, the rate of colon polyps increased more dramatically and was more dependent upon the length of antibiotic use. Those who took antibiotics for more than 2 months had a 1.69 times greater risk of developing colon polyps compared to women who hadn’t taken antibiotics. Because colon polyps can eventually lead to colon cancer, the findings are worrisome.

Additionally worrisome are the findings of a 2008 Finnish study of people ages 30-79 assessing their antibiotic use from 1995 through 1997. The researchers found that people who had six or more prescriptions of antibiotics in that two-year timeframe had a 15 percent increased risk of colon cancer.

A possible theory about why antibiotics may lead to the formation of colon polyps, and later cancer, could be because they indiscriminately kill healthy gut bacteria. As this occurs, other, more unhealthy bacteria predominate in the colon, which can affect the immune response in the colon, leading to disruptions in its lining and the formation of polyps.

So, yes, there does appear to be some increased risk of colon polyps with antibiotic use. I wouldn't be overly concerned about two courses of antibiotics, though I would be concerned, for a multitude of reasons, about a repetitive use of antibiotics. Such use raises the need to look for new ways to prevent, and treat, infections.

Robert Ashley, MD, is an internist and assistant professor of medicine at the University of California, Los Angeles.

Ask the Doctors is a syndicated column first published by UExpress syndicate.