Men taking drugs for sexual potency showed almost triple the rate of sexually transmitted diseases compared with those not taking the medications, a Harvard University study found.

The results, from an analysis of the health insurance claims of men aged 40 and older, may have more to do with the nature of the men using the impotence drugs than with the medicines leading them to have riskier sex, the research report said. The study, looking at men taking Pfizer’s Viagra and Eli Lilly’s Cialis, was published Monday in the Annals of Internal Medicine.

The higher rate of infections was seen in the year before and after the men started taking the prescription medicines, according to the analysis. That suggests that users of drugs to treat erectile dysfunction, which also include Bayer’s Levitra, may be more likely to engage in unsafe sex than nonusers, lead study author Anupam Jena said.