The lawsuit was brought by Michael Miller against Merck Frosst Canada, makers of Propecia and Proscar, two drugs that contain the medication finasteride.
Miller was 25 in 2008, when he said he obtained a prescription for Proscar from his doctor, according to a written court ruling.
Although that drug is for the treatment of prostate problems, it contains the same medication as Propecia, used to treat male pattern baldness.
Miller said he was advised to cut the five milligram tablet into four, making it roughly the same as the more expensive one-milligram Propecia tablets.
"Approximately one month after using Proscar the plaintiff alleges he experienced a diminished sex drive," said the March 28 written decision by B.C. Supreme Court Justice Robert Punnett posted Tuesday on the court website.
"Over the ensuing months, he became completely disinterested in sexual activity and was unable to maintain an erection."
Miller came across a website where other men linked sexual dysfunction to the drugs months after they had stopped taking them, and he discontinued use at the end of January 2009, "expecting that these alleged side effects would disappear."
They did not, the lawsuit claims, and Miller launched the class-action.
"The plaintiff asserts that the defendants were aware of the long-term side effects and that the warnings given in Canada were inadequate," the judge noted in his ruling.
In Sweden, for example, officials asked Merck six years ago to include in the warning the possibility of persistent erectile dysfunction continuing after use discontinued, and Merck agreed in 2008 to do so.
While the warning labels in Canada for both products did note that "so-called side effects," which were "uncommon and do not effect most men," may include impotence, problems with ejaculation and decreased semen, they were not enough, Miller claimed.
Punnett ruled that the class-action lawsuit is appropriate, but certification is pending. The judge asked Miller's lawyers to provide an affidavit with information on potential litigants and asked both parties to provide information on expert reports they plan to submit.
Merck has argued in this case and others that the plaintiffs have not provided scientific or medical evidence proving a link between the medication and the condition.
Neither a company spokesperson nor David Klein, Miller's lawyer, were available for comment Tuesday.
Miller's lawyers told the judge they have been contacted by almost 300 men — 55 of them from B.C. — who want to join the lawsuit.
A similar lawsuit was filed in 2011 in Ontario Superior Court by Sean Ramsaran and Chris Asimakopoulos.
In its third quarter earnings report last November, Merck & Co. announced it had been named in 265 lawsuits in New York and New Jersey alone over the sexual side effects of Propecia.