Tests show the anti-impotence drug may stop age-related macular degeneration. This follows a two-year trial led by scientists at Columbia University in New York. (Stock photo)

It sounds difficult to believe, but Viagra could restore sight to the blind.

Tests show the anti-impotence drug may stop further loss of vision for patients being robbed of their sight – and could even repair damage that’s already been done.

A two-year trial led by scientists at Columbia University in New York suggests the little blue pills could stop age-related macular degeneration, or AMD, in its tracks.

The condition is the UK’s leading cause of blindness, with about 600,000 Britons already thought to have suffered some loss of vision as a result of it.

Around 90 per cent of cases involve dry AMD, a form of the disease which comes on slowly over several years. The rest involve wet AMD, which can cause blindness in as little as three months.

Dry AMD usually develops after the age of 50 and is caused by the growth of new blood vessels over the macula, a small oval-shaped area at the back of the eye that helps us pick out visual details clearly.

These blood vessels leak fluid, causing scar tissue to form and destroying vision in the centre of the eye – making it difficult to recognise faces, read or watch television.

Recent research has found the condition is partly caused by reduced blood flow to the choroid, a vital layer of tissue that sits in front of the retina – and some small earlier studies had suggested Viagra can improve blood flow to this tissue.

Recent research found the condition is partly caused by reduced blood flow to the choroid, a vital layer of tissue that sits in front of the retina. Studies have suggested Viagra can improve blood flow to this tissue. (Stock photo)

In the Columbia study, five elderly patients with AMD were given two Viagra pills a day for two years.

The results, published in the journal Ophthalmologica, showed the drug improved vision for one participant and completely halted deterioration for the others.

Some drugs can already slow progression of AMD and increase vision in some cases, but the medicines have to be injected into the back of the eye every month.

The researchers said: ‘Viagra offers significant potential for vision retention and recovery. It is notable that patients remained visually stable and there was a significant improvement in vision in one participant.’

Professor Sobha Sivaprasad from the Royal College of Ophthalmologists said the Viagra findings were encouraging, although the study was small.

‘We now need bigger studies to replicate these findings before Viagra can be used as a treatment,’ she cautioned.

Previous studies have suggested Viagra could help treat a string of ailments, from heart attacks and lung disease to dementia.

The drug became available over the counter in Britain for the first time in March.