Viagra may cause vision impairment in men, a new study warns. 

Researchers from the University of New South Wales in Australia claim that the active ingredient in the anti-impotence drug may cause long-term damage to the vision of men who carry a common mutation for eye disease. 

The active ingredient, known as sildenafil, inhibits an enzyme that helps transmit light signals from the retina to the brain. The research team, led by Dr. Lisa Nivison-Smith from the UNSW School of Optometry and Vision Science, studied the effects of a single dose of sildenafil on both healthy mice and mice with a single copy of a mutant gene.

The gene mutation that causes blindness relates to the enzyme PDE6. People who have two copies of the mutant gene have the disease Retinitis Pigmentosa, which causes blindness. People with just one copy of the gene mutation have normal vision. Sildenafil treats erection by inhibiting the enzyme PDE5. However, it can also affect PDE6.   

The paper, which was published in the journal Experimental Eye Research, claims that sildenafil treatment caused normal mice test subjects to lose their visual function temporarily. In mice with retinal disease, the effect was heightened. The team found early signs of cell death in test subjects treated with sildenafil. This means that the chemical hastened degeneration for mice with impaired vision. According to the research team, the response time to sildenafil treatment was also much longer in mice with retinal disease. 

"Side effects can include sensitivity to bright light, blurred vision and altered colour vision," Dr. Nivison-Smith said in a press release.

"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."

In the clinical trials for Viagra, it was already revealed that high doses of sildenafil can cause vision disturbances even in healthy people. 

"These finding 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," Dr. Nivison-Smith added.

The team is conducting further research to find out how sildenafil caused long-term effects in carrier mice.