Viagra could cause blindness in men with eye problems, and even those with seemingly normal vision, a new study has found.
Australian researchers say an ingredient in anti-impotence drugs may permanently affect the vision of men with an inherited eye condition.
It could also affect those with normal sight who carry one gene which is linked with developing eye problems.
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Viagra could cause blindness in men with inherited eye condition retinitis pigmentosa - or those with normal sight who carry the gene for the condition, Australian researchers have found
One theory is that erectile dysfunction drugs may obstruct an enzyme which is important for transmitting light signals from the retina to the brain.
It is already known that sildenafil, the active ingredient in the erectile dysfunction medication, can cause temporary vision problems in some healthy people.
Doctors have now warned it could also cause permanent damage to the eyes of people retinitis pigmentosa.
People with normal sight that carry the gene for the condition may also be at risk, experts warn.
Retinitis pigmentosa is a rare hereditary condition that causes the cells in the retina - the area of the back of the eye which detects light - to degenerate and die.
This can lead to difficulty seeing in dim light, loss of peripheral vision, and sometimes blindness.
About one in 50 people are likely to be carriers of genes which lead to the degeneration of cells in the retina.
In the new study, researchers from the University of New South Wales, Australia, tested sildenafil on healthy mice and mice with one copy of this mutant gene.
They found the healthy mice experienced problems with their eyes for about two days.
However the drug affected the vision of mice who carried the mutant gene for two weeks.
The researchers also found early signs that the cells in the mice's eyes were dying, suggesting sildenafil may cause loss of vision in people who carry the gene for the disease but have normal vision.
Dr Lisa Nivison-Smith, of the UNSW School of Optometry and Vision Science, said: 'If cells actually die in the retina that would lead to blindness.'
People who have normal vision, but who carry a single copy of the mutant gene for retinitis pigmentosa, could be more susceptible to changes caused by sildenafil, she added.
The study was published in the journal Experimental Eye Research.The news comes after reports that Britain is braced for a 'Viagra boom' after the drug's patent ran out last year.
The cost plummeted by 93 per cent as cheaper generic versions of the drug became available.
Previous studies have linked Viagra and other anti-impotence drugs to side effects such as deafness.
American researchers also found that men who used Viagra increased their risk of developing skin cancer.
Retinitis pigmentosa is a rare hereditary condition that causes the cells in the retina - the area of the back of the eye which detects light - to degenerate and die
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