Pfizer is weighing up the possibility of selling an over-the-counter version of its blockbuster erectile dysfunction drug Viagra, according to media reports, as it bids to keep one step ahead of the competition.

The news was first revealed by UK magazine The Business and was confirmed by Reuters which received a statement form the New York-based drugs giant which read: "As with many of our products, Pfizer has routinely evaluated a number of options (for Viagra), including different formulations, new indications, over-the-counter, etc., and continues to do so."

Viagra (sildenafil) is still selling well, posting third-quarter sales of $423 million but its market share is being reduced by Eli Lilly and Icos Corp’s Cialis (tadalafil) which acts for up to 36 hours as opposed to the four hours that Viagra is effective.

Pfizer’s worldwide patent for the ED drug expires between 2011 to 2013, though it has been battling in China’s courts to defend it, and apart from competition from Cialis and Bayer/GlaxoSmithKline’s Levitra (vardenafil), the company has also seen counterfeit versions of the treatment become even more widely available on the internet. It is therefore not surprising that it is looking at ways of managing the product lifecycle of Viagra and the OTC option may make sense.

Nevertheless, Pfizer could have a tricky time persuading regulators of the virtues of an OTC version of Viagra, especially in terms of whether patients can treat themselves safely without medical advice. The drug’s label carries a warning that it must not be used with nitrates and that some patients may suffer affected vision or even priapism.

Reuters also reported Pfizer as saying that The Business article’s claim that the company was planning an oral-spray version of Viagra, which was being developed by US drug delivery firm Novadel, was unfounded.