ONE in four medications may not work properly if taken with St John’s wort or echinacea because they cause the drugs to be moved out of the body too quickly, research suggests.

A study by an American pharmacologist indicates that the two herbal extracts, often taken for depression and to boost the immune system, increase activity of an enzyme involved in the processing of many drugs.

Metabolising medicines too slowly or too quickly can cause drug toxicity and loss of therapeutic function. Drugs known to be affected include oral contraceptives, blood-pressure pills and drugs to prevent the rejection of transplanted organs.

Detailing the latest research at a conference in San Francisco yesterday, Christopher Gorski, of the Indiana University School of Medicine, said that more attention to metabolic considerations in the action of herbal medicines was needed. Dr Gorski found that both St John’s wort and echinacea increased the activity of a specific enzyme found in the liver and intestine.

He identified the enzyme, cytochrome P450 3A4, as the key to the accelerated metabolism by analysing patients who had been taking St John’s wort before receiving midazolam, a relaxant often given before minor surgical procedures.

Analysis indicated that the relaxant metabolised much more quickly in patients who had been using the herbal preparation. The team found similar effects on the Pill, which was also cleared from the body rapidly by the enzyme.

When 12 women who had not been taking St John’s wort added it to their drug regimen for two months, more than half experienced increased breakthrough bleeding, a clinical indication of decreased protection against pregnancy with the oral contraceptive.

Dr Gorski began his research with echinacea, a herbal preparation often promoted for its ability to prevent or treat colds and flu and a common herbal preparation. While research has suggested the acceleration of drug metabolism by St John’s wort, there has been little similar evidence linked to echinacea.

But when Dr Gorski administered the recommended doses for eight days to people taking various medication, he found that the herb altered the metabolic capacity of enzymes that play important roles in the breaking down of these medications. He is now looking at echinacea’s impact on other commonly used pharmaceuticals.

Dr Gorski told the conference, organised by the American Society for Pharmacology and Experimental Therapeutics, that his team was also studying the impact of St John’s wort on antihistamines and other pharmaceutical drugs.

He said that patients and clinicians should be aware of possible reductions in the therapeutic efficacy of prescription and over-the-counter drugs when taken at the same time as St John’s wort, echinacea and possibly other herbal preparations.

Of the 10,000 deaths caused by adverse reactions to prescription drugs every year in the UK, a significant number are thought to be caused by patients mixing their treatments with complementary medicines.

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