A popular herbal remedy is potentially dangerous for Aids and HIV patients and those who have had heart transplants, according to scientists.

St John's wort was found to interfere with two types of drug in separate studies carried out in the US and Switzerland.

Though clinical tests have shown that St John's wort can help treat depression and anxiety, there is a lack of information about its reaction with other drugs.

The latest studies, published in The Lancet medical journal, question its safety when combined with an HIV protease inhibitor called indinavir and the drug ciclosporin, used by heart patients who have had transplants.

St John's wort was found by Stephen Piscitelli at the US National Institutes of Health to reduce concentration of indinavir in blood plasma, when tested on healthy people. For HIV patients this could cause drug resistance and treatment failure, he said.

And Dr Frank Ruschitzka at University Hospital, Zurich, reported cases of patients who had undergone heart transplants and were readmitted to hospital after taking St John's wort to relieve mild depression.

Subsequent cardiac biopsies showed acute heart transplant rejection. Withdrawal of St John's wort led to a return to normal ciclosporin levels and no further rejection.

Misconception

Stephen Piscitelli said: "There is a misconception that herbal products like St. John's wort are safe, but this study demonstrates that there can be dangerous interactions when taken with other drugs prescribed to treat medical conditions.

"It is important for patients to tell their health care providers about their use of herbal products and complementary medicines."

Dr Ruschitzka added: "The common misconception that herbs always equate with no risk must be effectively disproved. Authorities should reconsider whether herbal medicines, as conventional drugs, need regulation and should be tested for safety, tolerability and efficacy for the benefit of our patients."

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.

She said: "Herbalists have realised St John's wort does have an effect on liver function, so it is not surprising that this is the case.

"The group of patients we are talking about here are transplant patients - they should always discuss any supplement regime they are using with their doctor."

She said the doses used in the Aids trial were three times higher than over the counter levels and the tests were done on healthy people, whose liver function would be different to that of HIV and Aids patients.