Finasteride (brand name Propecia), a drug often prescribed to fight hair loss, has been known to cause sexual side effects in some men who take it. What’s now coming to light is that those side effects may last an unexpectedly long time—for months or even years, according to a George Washington University study published in the Journal of Sexual Medicine and reported by ABC News.

Propecia, which works by preventing the body from converting testosterone to the related sexual hormone dihydrotestosterone, has a number of known side effects. These include decreased sex drive, erectile dysfunction, problems with orgasms and shrinking or painful genitals, as well as psychological effects such as anxiety, depression and mental fogginess.

For the study, researchers interviewed 54 men younger than 40 from PropeciaHelp, an online forum frequented by men who’d experienced side effects from the drug. They found that for 96 percent of the participants, the sexual side effects of Propecia lasted for over a year after they stopped taking it.

“Our findings make me suspicious that this drug may have done permanent damage to these men,” said study author Michael Irwig, MD. “The chances that they will improve? I think it’s lower and lower the longer they have these side effects.”

The researchers also suggest that doctors prescribing Propecia may not be aware of these negative effects. While the Food and Drug Administration approved Propecia in 1997, the sexual side effects have only been indicated on the label since 2011.

Irwig estimates that around 3 percent of men who take Propecia will suffer long-term sexual side effects. “But because the medication is prescribed so commonly,” he said, “it’s still a lot of people, likely several thousand men around the world.” He posits that there may be a genetic factor involved, but currently there’s no way to tell whether someone taking the drug will experience these effects.

Class-action lawsuits are currently under way against Merck, the manufacturer of Propecia, based on the drug’s side effects. Merck denies any link between Propecia and long-term sexual dysfunction.

To read the ABC News story, click here.