It is just the latest report of side effects associated with the herb and its active ingredient hypericin.

Joan Roberts, of Fordham University, New York, showed in laboratory experiments that the drug reacts with light, both visible and ultraviolet, to produce free radicals, molecules that can damage the cells of the body.

These, the scientists found, can react with vital proteins in the eye.

Roberts said: "If the proteins are damaged, they precipitate out of solution and make the lens cloudy. That's what a cataract is."

Cataract operation straightforward

If left to grow worse for long enough, cataracts can cause blindness. A simple operation is required in most cases to removed the damaged tissue and restore sight.

The photo-sensitivity caused by St John's wort is a well-known side effect.

Some users can develop rashes when exposed to strong sunlight, and, despite study evidence that the herb can alleviate the depression associated with Seasonal Affective Disorder, the combination of light-boxes, another treatment, and St John's wort is not recommended.

There have also been reports of intense pain developing in areas exposed to sunlight, which researchers believe is also due to damage to nerves caused by free radical molecules.

Ivor Roots, a clinical pharmacologist at Humboldt University in Berlin, said: "One should not assume that herbal drugs are without potential side-effects.

"In Germany, it is recommended to avoid tanning beds while taking St John's wort."

St John's wort may increase the effect of skin cancer treatments

This is not the only side-effect which has been reported. Minor side-effects such as upset stomach, allergic reaction, fatigue and restlessness are possible.

However, some doctors are sure the product is safe. Dr David Wheatley, a psychiatrist at the Charter Chelsea Clinic said he had had no complaints about side-effects.

He said: "It works extremely well for people with mild to moderate depression."

Used in cancer fight

However, the photo-sensitive effect may be harnessed by cancer experts to help tackle skin cancers by making them more sensitive to laser treatment.

A study is already underway to see if this is possible.

A spokeswoman for the Institute of Medical Herbalists said: "It is always a matter of dosage.

"People think that because it is a herbal remedy, if they take double the dose stated on the packet, they will get double the benefits with no side effects.