Every second, nine Viagra tablets are dispensed worldwide. (That’s about 73 miles of supersized erections a day.) But the little blue penis pill and its newer competitors, Levitra and Cialis, don’t play well with some other drugs. If used with poppers and certain nitrate-containing heart meds, these erectile enhancers can be deadly—drastically lowering blood pressure and increasing the risk of heart failure.

What’s more, protease inhibitors boost Viagra blood levels, causing an even bigger bang. “Giddyup!” you say? Slow down—a persistent hard-on can be painful and damage the penis, so point it toward the nearest emergency room. And talk with your doctor before you take Viagra or any other erectile aid.

Because of the drug interactions, some HIVers prefer to bone up with herbal alternatives advertised on the Net or in men’s magazines. However, going au naturel may not be so natural—or safe.

Researchers at two Canadian hospitals analyzed seven easily available “herbal” erection aids and found that two contained high levels of Viagra and Cialis. Since these compounds don’t occur naturally, the team concluded that the herbal products were deliberately contaminated with the drugs. Lead investigator Neil Fleshner, MD, says that “these findings seriously challenge the concept of safety of complementary and alternative agents.” So before taking any herbal remedy, do some research of your own to avoid getting, er, stiffed.