Jeff O. was 35 when his wife first confronted him about their increasingly anemic sex life. The couple had gone from almost daily romps to something like once every two months. "I had barely noticed," Jeff says. "I could have an erection. But it wasn't nearly as hard as before. And the other thing-" There's more? "Yeah," says Jeff, a medical sales rep from Southern California. "The color of my ejaculation turned yellow."

After multiple visits to the doctor and a battery of tests, Jeff learned he was suffering from low testosterone, and the likely cause shocked him. "I'll be on hormone therapy for the rest of my life," he says, "because of Propecia."

Propecia was hailed as something of a miracle drug when it first hit the market in December 1997. The drug was more effective at combating hair loss than Rogaine and didn't require messy topical application. At the time, pharmaceutical manufacturer Merck warned that a small percentage of all Propecia users could experience wilting erections and decreased libido. But to the millions of people who turned to Propecia to prevent their hair loss, it was well worth the risk.

Now, new research shows that Merck may have underestimated the persistence of some of Propecia's side effects. As part of a study published in a June 2011 issue of the Journal of Sexual Medicine, Dr. Michael Irwig of George Washington University surveyed 71 regular Propecia users to find that 94 percent suffered low libido, 92 percent experienced erectile dysfunction, and 92 percent had decreased sexual arousal. What's more, the survey showed that some side effects could last an average of 40 months after patients discontinued use of the drug, bucking the commonly held perception that the cure for any unwanted symptom is simply to stop taking the pill. "That's the novelty of this study – the persistent side effects of Propecia," Irwig says.

It's important to keep in mind that the sample size of Irwig's study was small and that most of the men he interviewed were recruited from Propeciahelp.com, an online support group. The percentage of users who suffer significant side effects is certainly smaller than this study suggests. Still, there's little doubt that these findings will attract more scrutiny in the future. They have already attracted lawyers: Several firms across North America are currently pursuing cases against Merck. The key question now is whether Merck knew that side effects could persist even after patients stopped taking the drug.

If there's a smoking gun here, it might just be in Merck's own hand. According to a complaint filed in Florida, Merck expanded the warning on the product label to include the "persistence of erectile dysfunction after discontinuation" in the face of a study by Sweden's Medical Products Agency suggesting that Propecia could be a long-term sexual inhibitor. But the rewrite appeared just in Sweden. In March 2010 the fine print was also amended in Italy, but nothing about the possibility of lasting sexual side effects appeared in the U.S. until this spring, perhaps in response to the publication of Irwig's study and a review from the Center for Sexual Function at the Lahey Clinic in Peabody, Massachusetts, which reached a similar conclusion.

In a statement responding to our inquiry, Merck acknowledged that some Propecia users – it put the number at 1.3 percent – experienced erectile dysfunction, but the company hedges on the subject of how long side effects can last, saying "a causal relationship...has not been established."

To understand the link between hair growth and sexual function, it helps to know how the drug works. Finasteride, the active compound in Propecia, is a 5-alpha reductase inhibitor, meaning it blocks the conversion of testosterone to the more potent dihydrotestosterone (DHT), which thins hair and contributes to baldness. Finasteride was originally developed to shrink enlarged prostates (in a drug called Proscar), but doctors noticed that it also grew hair.

It's worth noting that infant males born without the ability to convert testosterone to DHT are also born with ambiguous genitalia. "You need DHT to make a penis," Irwig explains. For this reason, pregnant women are told to avoid exposure to the drug.

The late-breaking warning on Propecia is cold comfort to David M., a 29-year-old from Virginia, who went off the drug after four years only to discover that his side effects worsened in the following 12 months. He now suffers from low testosterone levels (which can lead to gynecomastia, or what schoolyard bullies might call "bitch tits"). David acknowledges that he didn't do much research before filling his first prescription. "I was shedding hair, and my dermatologist suggested Propecia in conjunction with Rogaine," he says. "It definitely stopped the shedding. It did what it was supposed to do."

There are also many men who have been on the drug for years without incident. Abdulmaged M. Traish, a professor of biochemistry and urology at the Boston University School of Medicine, explains, "There is redundancy in biology. If one spark plug fails, others will continue to fire," meaning that the human body can compensate for the loss of DHT in a number of ways.

To adequately address the lasting effects of Propecia, Merck would need to conduct a study of 10,000 patients – half on Propecia, half on a placebo – over a five-year period, Irwig says. "But I don't see that study happening. It's too expensive." What we do know without a study is this: Stop taking Propecia and risk losing the hair you retained or regrew within the year.

What happens next is up to the courts; preliminary proceedings are already under way. (The FDA, meanwhile, has been relatively silent on the matter. It approved Propecia to fight hair loss in 1997 and, with the exception of a 2009 warning against Merck for not being up front about the hazards, hasn't really looked back.) But the one thing everyone can agree on is the need for more information. As Bernard Daskal, a partner at Lynch Daskal Emery, a Manhattan law firm investigating Propecia, says, "If you have questions, you should see a doctor, not a lawyer."

For what it's worth, Dr. Julian Mackay-Wiggan, a dermatologist at Columbia University, is still prescribing Propecia to her patients, as are many other doctors. "I say to my patients, 'This is a relatively new drug, and this is what we know about it, and the fact is that oftentimes we discover more as time passes,'" she says. "But right now, based on what we know, I feel comfortable prescribing it." If anyone doubts her intentions, she offers a fairly bulletproof defense. "I don't know if he'd want me to say this," she says, "but I have my husband on Propecia."