Australasian Science: Australia's authority on science since 1938

Viagra Could Harm Vision

The active ingredient in the erectile dysfunction drug Viagra can cause unusual visual responses in people who carry a common mutation for eye disease and have long-term detrimental effects on their vision.

Sildenafil can inhibit an enzyme that is important for transmitting light signals from the retina to the brain. Clinical trials of Viagra have found that high doses can cause transient disturbances in the vision of some healthy people.

“Side-effects can include sensitivity to bright light, blurred vision and altered colour vision,” says Dr Lisa Nivison-Smith of The University of NSW. “We are concerned that people who have normal vision but who carry a single copy of the mutant gene for the blinding disease retinitis pigmentosa could be more susceptible to these changes.”

A team led by Dr Nivison-Smith and Prof Michael Kalloniatis of UNSW studied the effects of a single dose of sildenafil on normal mice and mice with a single copy of the mutant gene. They reported in Experimental Eye Research that the normal mice had a transient loss of visual function after sildenafil treatment, but this effect was heightened and lasted longer in mice with the mutation. They also found early signs of cell death in the eyes of carrier mice, but not in the normal mice, suggesting that sildenafil may cause degeneration in carriers of retinal disease.

“These findings are highly significant because about one in 50 people are likely to be carriers of recessive genes which cause retinal disease, but are unlikely to know this because their vision is normal,” Nivison-Smith says.

Retinitis pigmentosa is the most common genetic disease that leads to blindness. It can be caused by a mutation in the gene that produces the enzyme PDE6. People with two copies of the mutant PDE6 gene get the disease, while carriers with just one copy have normal vision.

Sildenafil is part of a family of drugs that inhibit the enzyme PDE5 to treat erections, but they can also inhibit PDE6. This is likely to be a problem for carriers of a mutant PDE6 gene because they produce less of the enzyme than normal.

The researchers are conducting further studies to work out the mechanisms behind the long-term effects of sildenafil on the eyes of carrier mice.