Drug giant Pfizer has announced that it is cutting the cost of its erectile dysfunction drug Viagra in Canada to cushion the blow from generics entering the market ahead of schedule.

Viagra's (sildenafil) protection wasn't due to fall away until 2014, but this month a Canadian court ruled that its patent is invalid, thereby allowing cheaper, copycat forms of the drug onto the market.

The ruling by the Supreme Court came more than five years after Teva kicked off proceedings to challenge Viagra's patent. 

In response to the decision, Scott Wilks from Pfizer Canada said the firm is now lowering the drug's price "to be in line with generic versions because we are committed to ensuring that Viagra patients continue to have access to the original - made by Pfizer, and at a competitive price - simply by asking their pharmacist".

A four-pill pack of Viagra will now cost about $37 (Canadian) compared to the original $49, according to media reports.

Pfizer said it will spread news of Viagra's price cut in the country through a national print and online media campaign.