The US Food and Drug Administration has ordered that the labels of three top-selling impotence drugs be updated to highlight the risk of developing a rare form of blindness, but said that it was not possible to determine whether the drugs were actually to blame for this side effect.

The move was prompted by reports earlier this year that some patients taking Pfizer’s famed erectile dysfunction drug, Viagra (sildenafil), had developed a form of blindness known as nonanterior ischemic optic neuropathy, or NAION – a condition where blood flow is blocked to the optic nerve [[31/05/05a]].

As of the end of May this year, FDA data show that 43 cases of NAION had been reported amongst ED drug users - 38 for Viagra, four for Eli Lilly's Cialis (tadalafil) and one for Bayer/GlaxoSmithKline's Levitra (tadalafil) - in varying degrees of severity, including permanent vision loss.

Pfizer agreed last month to change the Viagra label to reflect the information, but was keen to point out that no causal relationship had been established and that NAION shares a number of common risk factors with erectile dysfunction: over 50 years of age, high blood pressure, high cholesterol, and diabetes [[28/06/05c]]. The label change also applies to Eli Lilly and Bayer/GlaxoSmithKline’s rival offerings.

The agency is advising patients to stop taking these medicines and contact a doctor immediately if they experience vision problems. They should also tell their doctor if they have ever had severe vision loss, as this puts them at an increased risk of developing NAION.

The announcement comes at a time when the US regulatory authority is coming under heightened pressure to communicate safety issues and investigate possible side effects. It has had its fingers burned several times in recent months with high-profile drug withdrawls.