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 Inc. Viagra and Eli Lilly and Co.’s Cialis, was published Tuesday in the Annals of Internal Medicine.

Men who took the impotence pills were almost three times more likely to have a sex disease, particularly HIV, the virus that causes AIDS, in the year before and after they started the drugs, according to the research.

The findings suggest that users of drugs to treat erectile dysfunction, which also include Bayer AG’s Levitra, may be more likely to engage in unsafe sex than nonusers, said Anupam Jena, the lead author of the study. Although sexual diseases are far more common in young people, infection rates are increasing in those middle-aged and older, the researchers 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.”

Indianapolis-based Lilly rang up $1.56 billion in Cialis sales last year, up 8 percent from 2008.

About 19 million new sexually spread infections occur each year in the U.S., 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 newly diagnosed 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 in a telephone interview.

The researchers looked at health insurance claims from 44 large U.S. employers from 1997, one year before Viagra was introduced, through 2006. They examined the claims of men over the age of 40 to see if they were prescribed erectile dysfunction drugs, finding 33,968 who used the impotence pills and about 1.38 million who did not.

The research found that in 2006, 3.6 percent of the men used Viagra from New York-based Pfizer; 1.7 percent took Cialis from Lilly, and 1 percent were getting Levitra from Leverkusen, Germany-based Bayer.

The authors weren’t able to determine how many of the men were married and how many were heterosexual, and other information on sexual behavior was unavailable.

When researchers compared men getting the drugs with men who were not, they found that sexually transmitted disease rates were higher in the first group both in the year before taking the pills and the year after.

The study found that users of ED drugs had an overall STD rate of 214 per 100,000 people in the year before they took the pills. That gave them a 2.8 times greater risk of developing a sexually spread infection than men who didn’t take the drugs.

The difference decreased slightly in the year after, when pill-takers had a 2.65 times higher risk than non-takers, the study showed.

The risk of getting HIV in the year before taking the pills was 3.32 times higher in drug-takers and 3.19 times greater in the year after, compared with those not taking the pills, they said. Users of the medicines also had higher rates of chlamydia.

The rates of HIV may be higher than other infections because the men seek treatment from their doctor for HIV symptoms like fever and weight loss, Jena said. Men may go to free clinics to treat other sexually transmitted diseases like herpes and gonorrhea, he said.

Doctors should advise older patients to remember safe-sex precautions if they ask for erectile dysfunction drugs, wrote Thomas Fekete, professor of medicine at Temple University in Philadelphia, in an editorial accompanying the study.

“STD counseling should not stop at age 40,” Fekete wrote.