Adam likes to think of himself as a “safe-sex ambassador.” The 34-year-old San Francisco resident, a successful professional (who does not want his real name used), volunteers at an AIDS counseling center, advising other gay men about safe sex and drug use. But he doesn’t always practice what he preaches. At a recent party, Adam took crystal methamphetamine, an illegal stimulant that creates overwhelming sexual desire but also constricts blood vessels, making it impossible for most men to have an erection. So Adam popped a Viagra pill he had purchased over the Internet and spent the next several hours having vigorous, unprotected sex with other partygoers. “The whole time, I’m saying, ‘this is stupid, why am I doing this?’” says Adam. Months of nail-biting followed, while Adam waited for tests to confirm he hadn’t been infected with the AIDS virus.

Adam isn’t alone. Since Viagra was introduced in the United States in 1998 to treat erectile dysfunction, the little blue diamond has also become a recreational favorite among gay men—as well as teenage “ravers”—who use it to enhance the effects of street drugs such as speed, ecstasy (used with Viagra, it’s called “Sextasy”) and amyl nitrate, or “poppers.” The consequences can be deadly: Viagra taken with poppers can lead to heart failure. Health officials in San Francisco say illicit Viagra use is contributing to surge in infection rates for HIV and sexually transmitted diseases. In Miami, the mixture of Viagra and ecstasy is known as hammerheading, for the pounding headache—and the prolonged, painful erection—it can produce. The city’s emergency rooms receive patients each weekend suffering from both.

A study of the gay community in San Francisco out next month finds that nearly a third of gay men surveyed report using Viagra. (Among those with HIV, the figure is 57 percent.) More than half of Viagra users say they get the drug from someone other than a doctor—and nearly 40 percent combine Viagra with illegal drugs. “The risk comes from enabling people who are in an altered mental and otherwise could not perform sexually to engage in high-risk sexual behavior,” says Dr. Grant Colfax, a researcher in the city’s AIDS unit.

At Pfizer, the New York pharmaceutical company that manufactures Viagra, officials take a dim view of the surging recreational popularity of their product. But while the company has cooperated with law enforcement to crack down on illegal Internet sales, it rejected a request from officials in San Francisco to put a warning label on Viagra about high-risk sexual behavior. “This is a public-health issue that needs to be addressed by public-health campaigns on safe-sex practices, [not] by focusing on one drug,” says spokesman Geoff Cook. Meanwhile, it will be up to men like Adam to warn others away from a dangerous—but tempting—pleasure.