Pfizer is talking with the US Food and Drug Administration about changing the labelling for its erectile dysfunction drug, Viagra (sildenafil), to include a warning that it may cause blindness.

However, the company insists that the form of blindness being linked to its drug – nonanterior ischemic optic neuropathy (NAION) – does not occur any more frequently with Viagra than in the general population. The risk factors for NAION – a type of stroke affecting the eye – are similar to those for ED, which typically affects men over 50 with concomitant illnesses such as high blood pressure, high cholesterol and diabetes.

“Most of the reported cases in which NAION has occurred in men taking Viagra have involved patients with underlying anatomic or vascular risk factors associated with the development of NAION,” said Pfizer in a statement.

Nevertheless, the bad press surrounding the link could help rival ED treatments such as Cialis (tadalafil) from Eli Lilly/ICOS and Bayer’s Levitra (vardenafil), although the possibility that blindness could be a rare class effect cannot be discounted. All three drugs are PDE-5 inhibitors, improving erections by constricting blood vessels and according to the FDA, 38 people who took Viagra and four who took Cialis have gone blind.

Pfizer still claims that Viagra is the market leader in the ED market, with $438 million dollars in first quarter 2005 sales [[20/04/05a]]. Meanwhile, Cialis achieved revenues of $150 million in the first quarter [[19/04/05a]], while Levitra is thought to be lagging in third place.

But all three drugs are meeting resistance from senators and taxpayer groups in the USA after it emerged that the bill for treating ED could be as much as $2 billion in the next decade [[18/05/05f]].