In fact, 75 percent of cancer patients who used complementary and alternative medicine didn't tell their physicians, according to a 2005 survey.

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The herb can cause headaches or tiredness the morning after, according to the National Institutes of Health's National Center for Complementary and Alternative Medicine.

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The study, which appears in the Annals of Internal Medicine, was funded by the National Center for Complementary and Alternative Medicine (part of the National Institutes of Health) and the Robert Wood Johnson Foundation.

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Ginkgo supplements were no better than a placebo for preventing dementia, according to the study, which was sponsored by the National Center for Complementary and Alternative Medicine and the National Institute on Aging of the National Institutes of Health.

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Dr William Van't Hoff, a consultant paediatrician and a spokesman for the Royal College of Paediatrics and Child Health, says this is an important, well constructed study that demonstrates "a high and unacceptable rate of adverse events" associated with the use of complementary and alternative medicine (CAM).

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He called on the government to "do nothing to discourage these courses, many of which are of a very high quality and give assurance to the millions of people who benefit in this country from alternative and complementary medicine".

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Lord Henley told peers that the government was "neutral" on the issue of alternative and complementary medicine and left "all decisions on commissioning and funding it to the NHS".

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