But his symptoms only got worse. He was demoted from his job as an investigator for the Department of Homeland Security in Texas after his symptoms were evaluated by a psychiatrist, who pronounced him unfit for duty. Rossello said the psychiatrist was unfamiliar with the neurological side effects of Propecia.
Abdulmaged Traish, a professor of biochemistry and urology at Boston University School of Medicine, said scientists are just beginning to investigate finasteride's effects on the brain and nervous system.
"We are just beginning to venture in that direction. It might be in the next five to 10 years that we may find there's more to this story than we know now," Traish said.
He said he would not advise men to take this drug to treat a cosmetic problem like hair loss.
"Why take that chance? Telling the risks of the drug is fine, but most physicians don't sit and take the time to explain to their patients the possible effects of these drugs."
On its website, the FDA said only 36 of 945 men who took Propecia in clinical trials reported any adverse sexual side effects. Irwig agrees that the number of men who will experience these long-lasting side effects is relatively small, likely around 3 percent of all men who take the drug.
"But because the medication is prescribed so commonly, it's still a lot of people, likely several thousand men around the world," he said.
And currently, doctors have no way of knowing which patients will suffer the long-term side effects. Irvig said it's possible that an unknown genetic factor drives how individual men respond to the drug.
Both Malley and Rosello were not involved in Irwig's study, but they are two of many men who have joined class-action lawsuits against Merck over the side effects of Propecia. In May, Malley camped out in front of Merck's headquarters in New Jersey for a week, staging a hunger strike to protest the company's response to Propecia's side effects. A representative came out to deliver a letter to him, detailing how he could report his symptoms to the company.
Malley has a simple message for men who might consider taking Propecia for their hair loss.
"Stay away from it at all costs," he said.