Many common herbal supplements, including green tea and ginkgo biloba, may interact with prescription drugs, according to a new review of research published in the British Journal of Clinical Pharmacology. These interactions can make the drug less effective and can even be dangerous or fatal.
Doctors know that herbs can influence treatment regimens, researchers from the Medical Research Council of South Africa write in a new paper. But because people typically don’t tell their healthcare providers what over-the-counter drugs and supplements they’re taking, it’s been difficult for scientists to keep track of which drug and supplement combinations to avoid.
The new review analyzed 49 reports of adverse drug reactions and two observational studies. Most of the people in the analysis were being treated for heart disease, cancer, or a kidney transplant and were taking warfarin, statins, chemotherapy drugs, or immunosuppressants. Some also had depression, anxiety, or a neurological disorder and were treated with antidepressants, antipsychotics, or anticonvulsants.
From these reports, the researchers determined that the herb-drug interaction was “likely” in 51% of the reports and “very likely” in about 8% of the reports. About 37% were classified as possible herbal drug interactions, and only 4% were considered suspicious.
In one case report, a patient taking statins complained of severe leg cramps and pain after drinking three cups of green tea a day, which is a common side effect. The researchers wrote that this response was due to green tea’s effect on blood levels of statins, though they said more research is needed to rule out other possible causes.
In another report, the patient died after having a seizure while swimming, despite taking regular anticonvulsant drugs to treat the condition. However, his autopsy revealed that he had decreased blood levels of these drugs, possibly due to the ginkgo biloba supplements he also regularly took, which affected their metabolism.
Taking herbal supplements has also been associated with worsening symptoms of depression in people taking antidepressants, and with organ rejection in people who have had kidney, heart, or liver transplants, the authors write in the article. For cancer patients, chemotherapy drugs have been shown to interact with herbal supplements, including ginseng, echinacea, and chokeberry juice.
The analysis also showed that patients taking warfarin, a blood thinner, reported “clinically significant interactions.” Researchers speculate that these herbs may interfere with the metabolism of warfarin, thereby reducing its anticoagulant capacity or causing bleeding.
The authors say more lab studies and closer observations in real people are needed to provide stronger evidence for interactions between specific herbs and drugs. “This approach will inform drug regulatory authorities and pharmaceutical companies to update label information based on available data to avoid adverse side effects,” they wrote.
He also reminds patients that they should always tell their doctors and pharmacists about any medications or supplements they are taking (even products sold as natural or herbal), especially if they have been prescribed a new medication.