(WASHINGTON) -- Men who take Propecia for baldness may experience sexual side effects that last for months to years, even after they stop taking the drug, a new study published Thursday in the Journal of Sexual Medicine suggests.
Researchers from George Washington University interviewed 54 men under the age of 40 who reported side effects for three months or more after taking Propecia, also called finasteride, to treat their hair loss. None of the men reported having any sexual, medical or psychiatric problems before they took the drug.
Some of the men took the drug for a few weeks, others took it for years, but all of them reported side effects such as erectile dysfunction, decreased sexual drive, problems with orgasms, shrinking and painful genitals, even some neurological problems, such as depression, anxiety and mental fogginess.
For 96 percent of the men, the sexual problems lasted for more than a year after they stopped taking the drug.
"Our findings make me suspicious that this drug may have done permanent damage to these men," said Dr. Michael Irwig, the author of the study. "The chances that they will improve? I think it's lower and lower the longer they have these side effects."
Irwig cautions that it's possible that only men who were the most affected by the drug participated in the study. Because he recruited his study participants through an online forum called PropeciaHelp, a group for men who have experienced persistent sexual side effects from the drug, he said the study may not have included men who have fewer or less pervasive side effects.
Finasteride works by blocking the conversion of testosterone into a more potent form, called DHT, which contributes to hair loss. It was originally developed in 1992 by drug giant Merck as a treatment for enlarged prostates and sold as the drug Proscar.
Propecia was approved by the U.S. Food and Drug Administration in 1997, and at that time Merck noted that a few men reported sexual side effects during clinical trials of the drug. On its website, the agency said those side effects were resolved when patients stopped taking the drug.
But the agency received more than 400 reports over 13 years from consumers reporting sexual dysfunction, and nearly 60 men reported that those side effects lasted longer than three months after the men stopped the medication. In 2011, the FDA mandated a label change for Propecia and Proscar, warning that some patients reported erectile dysfunction that lasted after patients stopped taking it; in April, the agency updated the label to include reports of libido, ejaculation and orgasm disorders.
In a statement, Merck said no evidence has proved a causal relationship between Propecia and long-lasting sexual dysfunction.
"Merck believes that Propecia (finasteride) has demonstrated safety and efficacy profiles and that the product labeling appropriately describes the benefits and risks of the drug to help inform prescribing," the company wrote in the statement.
But researchers say many physicians who prescribe finasteride are likely not aware that the side effects of the drug may haunt patients for years.
"These things just get handed out left and right for any urinary symptoms," said Dr. Ryan Terlecki, an assistant professor of urology at Wake Forest School of Medicine, who has prescribed Proscar for some of his patients with enlarged prostates.
Terlecki said the findings about long-term side effects from the drug are alarming, but more research will likely be needed before doctors can know for sure that the symptoms are completely attributed to the drug.
Copyright 2012 ABC News Radio