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.

"Younger people have more sex partners than older folks," said Jena, a medical resident in internal medicine at Massachusetts General Hospital and Harvard Medical School in Boston. "But per sexual encounter, the actual safeness of the sex is probably lower among older folks in the sense that they don't use condoms," he said.

About 19 million new sexually spread infections occur each year in the United States, almost half of them among people ages 15 to 24 years old, according to the U.S. Centers for Disease Control and Prevention.

Still, people aged 40 to 49 accounted for the largest proportion of new HIV/AIDS cases, 27 percent, in 2007, according to the CDC. Those 50 to 59 accounted for 13 percent, while those over the age of 60 accounted for 4 percent.

"Old folks can contract STDs and we need to be vigilant about it," Jena said. The research found that in 2006, 3.6 percent of the men used Viagra; 1.7 percent, Cialis, and 1 percent, Levitra.

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