Ophthalmologists are cautioning patients about visual side effects caused by Viagra, an oral therapy for impotence. Viagra relaxes smooth muscles in the penis by interfering with the action of a special enzyme. A nearly identical enzyme in the retina, the layer of light-sensitive cells lining the back of the eye, may also be affected by Viagra, causing a mild disturbance of color vision in approximately 3% of people taking a higher than recommended dose.

Because of this unusual side effect, doctors recommend people with retinitis pigmentosa (an inherited disease affecting the retina) use Viagra with caution. People with other retinal problems should discuss their condition with their ophthalmologist before taking Viagra.

Until more data is available, physicians strongly recommend people stay at the lowest dose possible. Presently 50 mg is the recommended amount.