Canada's top court has ruled that Pfizer's patent on the company's groundbreaking erectile dysfunction drug Viagra is void.

The Supreme Court of Canada sided Thursday with Teva Canada Ltd., a division of the Israeli-based Teva pharmaceutical empire, in a long-standing court battle against Pfizer over whether or not the latter company has the right to the patent in the first place.

Teva had argued that the details provided in Pfizer's original patent application (number 2,163,446, granted in 1998 and set to expire in 2014) was invalid because in it, Pfizer listed several different chemical compounds without ever specifying which one was the actual active ingredient — sildenafil.

Disclosure rules breached

Canada's Patent Act gives a company a 16-year monopoly on a product if it can prove it is a new invention. In return, the company must show publicly in its application how it created its product.

Lower courts had sided with Pfizer in the dispute until Canada's top court agreed, in a 7-0 decision, with Teva's argument.

Pfizer's patent on Viagra "does not meet the disclosure requirements set out in the act" Justice LeBel wrote in the ruling." I would therefore allow the appeal with costs and hold that Patent 2,163,446 is void."

"Pfizer gained a benefit from the act — exclusive monopoly rights — while withholding disclosure in spite of its disclosure obligations under the act," LeBel wrote.

"As a matter of policy and sound statutory interpretation, patentees cannot be allowed to 'game' the system in this way. This, in my view, is the key issue in this appeal," LeBel said.

Pfizer 'disappointed' with ruling

For its part, Pfizer said it was disappointed with the ruling.

"Pfizer expects to face generic competition in Canada shortly," the company said in a statement issued by its New York office. "Pfizer will continue to vigorously defend against challenges to its intellectual property."

The unanimous decision opens the door for Teva to introduce a generic version of Viagra. By the afternoon on Thursday, Teva had already moved to do just that, posting a message on its website, announcing the creation of Novo-Sildenafil and noting the product is available via prescription.

"Canadian consumers will be saving money on it. There will probably be other generics involved soon enough," said Richard Gold, an intellectual property expert at Montreal's McGill University.

Other competing products with different active ingredients such as tadalafil (known as Cialis) and vardenafil (Levitra) already exist.

The ruling sends a strong message to future patent applicants that "gaming" the system will no longer work, said Gold.

"This is particularly important in the pharmaceutical industry where both brand-name and generic companies play games, wasting court resources and putting money into litigation rather than into actual research."