Pfizer's legal monopoly on one of its top-selling drugs just got shredded in Canada. The Canadian Supreme Court has ruled 7-0 the company should have its patent taken away because the drug company attempted to "game" the system, grabbing a patent without disclosing what their invention really was.

Pfizer was able to acquire its Canadian patent without naming the compound required to make Viagra, namely, sildenafil citrate. The Canadian patent system, like all patent systems, is a kind of bargain between patentees, who are given a limited monopoly on a particular product or process, and the public, which is supposed to benefit from the disclosure of a new invention, the justices noted in their opinion.

"Pfizer had the information needed to disclose the useful compound and chose not to release it," the ruling said. "As a matter of policy and sound statutory interpretation, patentees cannot be allowed to 'game' the system in this way."

The company that successfully busted the Pfizer patent is Israel-based Teva Pharmaceuticals, the world's largest generic drug company. Generic Viagra will probably hit the Canadian market in short order and cut Pfizer's profit in that country to almost nothing.

Analysts who spoke to Reuters about the ruling noted that losing the Canadian market isn't really big enough to move the needle for a company as large as Pfizer. Still, Canadian sales of Viagra are about $80 million according to The Globe and Mail, and Pfizer will likely be saying goodbye to most of that revenue. The drug is Pfizer's sixth most selling medicine, with global sales of $2 billion.

In the US, Pfizer's patent rights on Viagra were originally set to expire in 2012. But when generic companies moved to enter the market, Pfizer piled on a "method-of-use" patent over the same drug, set to expire in 2019. A federal judge upheld that patent after a bench trial last year, so Pfizer will be the only company allowed to sell sildenafil in the US for at least seven more years, and prices will remain high.

Viagra sales in the US have still dropped off somewhat, because it faces stiff competition from other patented drugs such as Eli Lilly's Cialis.