In fact, they report in the Annals of Internal Medicine, the rate of sexually transmitted diseases (STDs) in older men taking erectile dysfunction drugs like Viagra is twice as high as in their non-medicated peers.
In both groups, however, the numbers are swelling. According to the Centers for Disease Control and Prevention, there were more than six new cases of STDs per 10,000 men over 40 in 2008, up almost 50 percent since 1996.
"Younger adults have far more STDs than older adults, but the rates are growing at far higher rates in older adults," said Dr. Anupam B. Jena of Massachusetts General Hospital in Boston, who led the study.
While the reasons for this development aren't well understood, he said more divorces and better health might have conspired to boost sexual prowess and activity among graying heads.
The problem, however, is that older adults appear to flout safe sex practices. For instance, the researchers note, 50-year-olds are six times less likely to use a condom than men in their 20s.
"We are typically unaccustomed to practice safe sex over the age of 50, because the risk of pregnancy is eliminated," Jena told Reuters Health.
To test whether the introduction of Viagra in 1998 might explain some of the STD surge, Jena and colleagues examined insurance records for more than 1.4 million U.S. men over 40. The average age in the study was about 60 years.
The most commonly found STD was HIV, followed by chlamydia, syphilis and gonorrhea.
Among the few percent of men who had filled prescriptions for erectile dysfunction drugs, more than two in a thousand had been treated for an STD in the year before they got the drug.
A year later, the number dropped to half that, suggesting that Viagra and its chemical cousins didn't fuel STDs.
However, the risk of contracting an STD turned out to be more than twice as high in men taking erectile dysfunction drugs compared with those who didn't.
"These users have a different sexual risk profile than non-users," said Jena, adding that the data didn't reveal any good explanation.
In an editorial, Dr. Thomas Fekete, of Temple University School of Medicine in Philadelphia, noted that it would have been valuable to know more about the frequency of sexual encounters, sexual partners and orientation.
He added that prevention strategies should still be directed at younger age groups, whose STD risk is at least 10 times higher than in middle-aged and older adults.
Still, he said, the authors remind us "that men older than 40 years remain sexually active, even if they need chemical assistance to do so. This study also serves as a reminder that sex after age 40 years is not necessarily safe."
Jena recommended that doctors take a few minutes to discuss safe sex with older men when they prescribe Viagra.
His advice? "Look, just realize that you are at higher risk for STDs, and try to be careful like you used to be 30 years ago."
Annals of Internal Medicine, online July 5, 2010.