"We would never encourage irresponsible marketing of St John's Wort, " he told the BBC.

BBC: Herb 'helps ease depression'

The flax-leaved St John's Wort is now spreading at Craig y Dre in Tremadog, according to Natural Resources Wales (NRW).

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So to discover that a plant like St John's Wort has pharmaceutical properties does not rock me back on my heels.

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The recent clinical trial the doses of the St John's Wort extract were similar in size to pharmaceutical doses of antidepressant.

BBC: 'Natural doesn't always mean best'

St John's Wort has been used for many years in folk medicine to treat disorders such as burns, wounds and depression.

BBC: Herb 'helps ease depression'

And since no patent can be issued on Ginko Biloba or St John's Wort, there is no advantage to any company which pays to test natural drugs.

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But Dr Ann Walker, senior lecturer in human nutrition at the University of Reading and a qualified medical herbalist, said the findings would make little difference to the average St John's wort user.

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