Elmiron (pentosan polysulfate sodium) can damage the retina, causing permanent vision problems if the drug has been taken at high doses and for many years. The problem is irreversible if it has been taken for 15 years or more.

Regular orange eaters are 60 per cent less likely to have developed the disease 15 years later, a new research study has discovered.

People who eat an orange a day are the least likely to suffer from macular degeneration when they're elderly, but even eating an orange once in a while also has some protective effect.

Although vitamins C, E and A are supposed to be keep our eyes healthy, it was the flavonoids specifically found in oranges that seemed to have the greatest benefit, say researchers from the University of Sydney.

Eating between 100 and 142 mgs of vegetable nitrates—found in leafy vegetables and beetroot—every day reduces the risk of developing AMD by around 35 per cent.

Spinach contains 20mg of nitrate, and beetroot has 15mg, per 100g serving.

It's well known the drug can cause vision problems, but they seem to be temporary and usually self-correct within 24 hours. But researchers at Mount Sinai Hospital have discovered that the damage can be permanent in people who take the drug for a long period or who overdose.

Although all of us need to be able to see colours, the drug's side effects can be a special problem for people who rely on colour recognition in their work, and they need to be especially careful before they start taking Viagra (sildenafil citrate), the researchers say.