In this video, Motley Fool Health-Care Analyst David Williamson breaks down just what happened in the recent Canadian Supreme Court ruling against Pfizer (PFE +4.59%). The Canadian patent on the company's drug Viagra was due to expire in 2014, but the Court struck down the patent in a unanimous ruling, invalidating the patent. Williamson explains the losers and the winners in this situation, and just how much we can expect Pfizer to really be affected.