Merck, which has major operations in the Philadelphia area, on Tuesday said through spokesman Ron Rogers: "We are aware of the lawsuits, and we intend to vigorously defend against them."
Finasteride was first approved in the United States in 1992 as Proscar, a treatment for benign enlargement of the prostate gland. A lower dose, Propecia, was approved in 1997 for male pattern baldness, but balding men often buy the higher-dose product and split the tablet to save money.
The litigation comes as a growing number of physicians, studies, and patient websites are warning of a small but significant chance of irreversible sexual effects, including impotence, from finasteride.
"My message is that clinicians who are going to prescribe this drug for hair loss should sit down and have a candid, serious discussion about the risks and benefits, and warn their patients that in some men - not all - these sexual side effects are persistent," said Abdulmaged M. Traish, a Boston University professor of biochemistry and urology who in December published a review of finasteride studies. "For some men, it's a life sentence."
Finasteride works by blocking the conversion of testosterone into an even more potent male sex hormone that is crucial to sexual development and function. The same potent hormone can attack hair follicles, so blocking it can interrupt hair loss.
Merck's U.S. labeling and website warn that some men suffer problems such as decreased libido, erectile dysfunction, and decreased ejaculate. However, the company says these problems are unusual - occurring in fewer than 2 percent of men on Propecia - and get better.
"These side effects decreased to 0.3 percent of men or less by the fifth year" on Propecia, the website says. "These side effects went away in men who stopped taking Propecia because of them."
In contrast, researchers - and affected men - say problems can persist despite youth, good health, and very brief use of the drug.
The plaintiffs in the U.S. lawsuit, for example, are Steven Rossello, 28, of Harlingen, Texas, and Justin Herrman, 31, of Las Vegas. The Canadian suit, filed in January by lawyer David Klein, is a class action initiated by Michael Miller, a Vancouver man in his early 20s.
On http://propeciahelp.com/, a forum for men with unresolved problems, one subscriber who took finasteride for only a week said his erectile dysfunction resolved after he stopped the drug but "testicular pain" was still a problem days later.
Michael Irwig, a George Washington University endocrinologist who specializes in male hormone disorders, turned to that website to conduct the first study of men with persistent problems.
The study, being published Friday in the Journal of Sexual Medicine, involved detailed interviews with about 100 healthy men worldwide.
"I focused on the younger guys who took it for hair loss," Irwig said. "I wanted to make sure they had no sexual problems before starting the medicine, and no history of depression."
Although Irwig refused to call the men's problems "permanent," a few are still suffering five or 10 years after taking finasteride.
He also does not have enough data to calculate the risk of long-term sexual effects, but he has a hunch.
"I think the risk of persistence is less than 1 percent, because in the medical literature, the risk of [temporary] sexual side effects is less than 5 percent," he said.
If the risk is indeed small, that may explain why it has not received much attention until now, he said.
Andrew Rynne, a physician in Kildare, Ore., posted a warning about Propecia on his website http://www.andrewrynne.com/ in October. He believes the fraction of men who suffer sexual side effects is higher than 2 percent - but even 2 percent is too high.
"To the uninitiated, it might seem like a low figure," he wrote. "But you are dealing here with a naturally occurring normal male phenomenon called 'male pattern baldness.' This is not an illness or a disease. If in an attempt to 'cure' it, you are getting a 2 percent rate of serious side effects, then that, quite frankly, is unacceptable."
Millions of men worldwide have taken finasteride. In 2009, Propecia sales were worth $440 million, while Proscar brought in $290 million.
In Sweden, the Medical Product Agency began investigating reports of persistent sexual dysfunction in 2006, and a TV station aired a story. In 2008, Merck revised the Swedish Propecia label to warn of "persistence of erectile dysfunction after discontinuation of treatment."
The same revision has since been made to labels in the United Kingdom and Italy.
In the United States and Canada, labeling was revised last year to add a warning about depression - but not persistent sexual effects.
Contact staff writer Marie McCullough at 215-854-2720 or firstname.lastname@example.org.