Herbal and complementary treatments may put lives at risk when they are mixed with conventional medicines, an expert has warned.

Research shows that unwanted side effects and health problems can be triggered by combining natural supplements with widely used drugs.

An estimated ten million Britons regularly take herbal remedies, vitamins and minerals.

But popular supplements such as garlic, ginger, St John’s Wort and even green tea can all have hazardous impacts on the effect of prescription or over-the-counter medicines.

Dr Catherine Ulbricht, an American pharmacist and author on herbal medicines, suggests that the danger from mixing is greatest in younger and older people and those with multiple health conditions who take numerous pills. Pregnant women and breastfeeding mothers could also be at risk.

Dr Ulbricht said doctors need to tell patients about any potential risks to avoid any harmful or life-threatening effects of taking natural supplements.

Writing for the magazine Alternative and Complementary Therapies, Dr Ulbricht said: ‘Natural does not equal safe. If something has a therapeutic action in a human body, this substance can also cause a reaction or an interaction.’

Her research found that there are serious risks when popular natural products such as feverfew, ginger, and ginkgo, which all contain nutrients that lower blood pressure or thin the blood, interact with aspirin and warfarin, which are taken by millions to ward off heart attacks.

Herbal medicines

Even mixing energy drinks or nutritional bars with medication can cause dangerous side effects.

Dr Ulbricht also warned that garlic supplements, which are taken to reduce blood pressure, can interfere with anti-clotting medications and the immunosuppressant drug cyclosporine which prevents transplant rejection.

She also highlighted the interaction of grapefruit juice with medicines, which suggests that it may block enzymes that normally break them down in the intestines.

One glass of grapefruit juice could persist for longer than 24 hours, meaning many people are advised not to drink it at all while also taking certain drugs, including statins, antihypertensives, psychiatric drugs and Viagra.

Dr Ulbricht adds that the sedative valerian, used as a natural alternative to sleeping pills, can intensify the effect of anaesthetics, while St John’s Wort, a herbal treatment for depression, can interact with immunosuppressive drugs and potentially lead to the rejection of transplants.

Surgeons advise that most surgery-related side effects can be avoided by not taking herbal products at least one or two weeks prior to surgery and during the postoperative period while prescription medications such as blood thinners are prescribed.