Generic Viagra is set to hit the market soon in Canada after the country's Supreme Court voted unanimously (7-0) that the patent on Pfizer's erectile dysfunction blockbuster is invalid.

The ruling comes more than five years after proceedings were initiated by Teva challenging the patent on Viagra (sildenafil). The Israel-based company was knocked back by Canada's Federal Court in June 2009 and had an appeal dismissed in September 2010.

However, the Supreme Court backed Teva this April and has now confirmed its decision, saying that Pfizer did not sufficiently disclose the exact compound that is effective in treating ED. The patent covering Viagra includes the statement that “one of the especially preferred compounds induces penile erection in impotent males”, but the application does not disclose that the compound that works is in fact sildenafil "or that the remaining compounds had not been found to be effective in treating ED", the court says.

Barry Fishman, chief executive of Teva Canada, said the firm "took the lead in bringing a generic form of sildenafil to consumers, despite repeated legal setbacks". However, he noted that companies like his "incur significant legal costs to challenge numerous weak or frivolous patents on many products and these investments ultimately benefit all payers and contribute to the sustainability of our healthcare system".

He added that through such litigation, "generics have generated cumulative savings for Canadians of more than C$20 billion compared to awaiting patent expiry". The Canadian patent on Viagra was scheduled to expire in 2014.

Pfizer said it now expects to face generic competition in Canada shortly, "is disappointed with the Court's ruling and will continue to vigorously defend against challenges to its intellectual property".