An active ingredient in the erectile dysfunction drug Viagra could cause unusual visual responses in people who carry a common mutation that leads to an eye disease and may have long-term detrimental effects on their vision, researchers warn.

 

 

 Sildenafil can inhibit an enzyme which is important for transmitting light signals from the retina to the brain.

 

 It is already known from clinical trials of Viagra that its use in 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," said Lisa Nivison-Smith from the University of New South Wales.

 

 "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," Nivison-Smith explained.

 

 A team studied the effects of a single dose of sildenafil on normal mice and mice with a single copy of the mutant gene.

 

 They found the normal mice had a transient loss of visual function after sildenafil treatment, but this effect was heightened in the mice with the mutation and the response lasted longer.

 

 "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," Nivison-Smith maintained.

 

 Retinitis Pigmentosa is the most common genetic disease which leads to blindness.

 

 The results were published in the journal Experimental Eye Research.