That little blue pill may have a big side effect: nonarteritic anterior ischemic optic neuropathy (NAION).

A recent report describes seven patients, ages 50 to 69, who had typical features of NAION within 36 hours after taking Viagra (sildenafil, Pfizer).

Six patients had vision loss within 24 hours after use of the drug. Final visual acuity in the affected eye ranged from 20/20 to light perception. Both eyes were affected in one individual.

Seven similar cases have been previously reported.

All affected individuals had pre-existing hypertension, diabetes, elevated cholesterol or hyperlipidemia. The researchers conclude that Viagra may provoke NAION in individuals with an arteriosclerotic risk profile.

All the patients also had a low cup-to-disc ratio. The researchers say that Viagra effectively acts as a blood vessel constrictor, which may cut off blood flow to the optic nerve.

The authors recommend that eye doctors should ask all men with NAION about the use of Viagra. They also caution patients who have a history of NAION in one eye that Viagra may increase the risk of developing NAION in the fellow eye.

Pomeranz HD, Bhavsar AR. Nonarteritic ischemic optic neuropathy developing soon after use of sildenafil (Viagra): A report of seven new cases. J Neuroophthalmol 2005 Mar;25(1):9-13.