Teva will have to wait eight years before it can market a generic version of Pfizer's erectile dysfunction drug Viagra in the USA, according to a US court.

The generic drugmaker has been trying to topple a method-of-use patent for Viagra (sildenafil) that expires in October 2019, but the US District Court for the Eastern District of Virginia decided that this should remain in force.

Teva had challenged the patent on the grounds that Pfizer had 'double-patented' Viagra in the USA - one expiring in March 2012 and the second in 2019 - and that the second use patent was invalid. It also argued that Pfizer withheld information from the US Patent and Trademark Office. Those assertions were dismissed by the judge in the case.

Teva has leave to appeal the ruling, has not said publicly whether it will do so. Meantime, litigation against other generic drugmakers accused of infringing the same patent remains pending, according to Pfizer, although no trial dates have been set.

Pfizer reported Viagra sales of $495 million in the second quarter of this year, a rise of 1%, and the brand is the drugmaker's sixth best-selling brand.

The product has already started to lose patent protection in some international markets - notably Brazil in June 2010 - with more to come by the end of next year, so reaffirming its US patent position is a big boost for the franchise. More than 50% of Viagra's sales are made in the USA, accounting for $1 billion a year in turnover.

Meanwhile, Pfizer is recognised as facing a steep patent cliff in the next couple of years, in common with many of its peers in Big Pharma. In addition to Viagra, the company has three more of the top 15 patent expiries in the US in the next two years, according to a recent EvaluatePharma report. The others are for top-selling product Lipitor (atorvastatin), the antiulcerant Protonix (pantoprazole), anti-psychotic Geodon (ziprazidone).

"We are pleased that the court recognised the validity and enforceability of our Viagra patent for the treatment of erectile dysfunction," said Amy Schulman, executive vice president and general counsel for Pfizer in a press release issued yesterday.

"Protecting the intellectual property rights of our innovative core is critical, and Friday's court decision acknowledges Teva's clear violation of our patent rights."

Pfizer's share price rose a little under 3% on the back of the announcement to close at $18.34 yesterday.