Many consider it a miracle beverage that can help shed pounds, reduce the risk of heart disease and even ward off some types of cancer.
But new research suggests that green tea may also have a dark side - by blocking the effects of a cancer drug.
Components of green tea seem to bind to the cancer drug bortezomib, also known by brand name Velcade, and stop it from reaching its target in cancer cells, according to a study published online yesterday in the journal Blood.
Velcade is primarily used to treat multiple myeloma, a type of blood cancer.
Health Canada approved the drug in 2005 for use in patients who have already received at least one prior therapy or a stem-cell transplant, or are not suitable for transplant.
"We expected the opposite. We expected that [green tea] would help the drug," said Axel Schönthal, professor in the Keck School of Medicine at the University of Southern California in Los Angeles and senior author of the study.
The discovery is also a strong reminder that herbal remedies aren't necessarily benign just because they occur naturally.
"The danger is that nowadays in every health-food store, you can buy these concentrated green tea extracts," Dr. Schönthal said. "All types of cancer patients, they read up on these alternative treatments. It is known that some of them do take different types of herbs and everything that is available in health-food stores."
Green tea is made from the leaves of the Camellia sinensis plant and has been used for thousands of years in traditional medicine. Health Canada has given companies that sell green tea approval to make three health claims about their products: Green tea is a source of antioxidants; it can increase alertness; and it may support or maintain cardiovascular health.
Studies examining whether green tea lowers the risk of developing cancer, cardiovascular disease or a host of other health problems have been contradictory.
But the new study sheds light on a potentially dangerous interaction that needs to be brought to the attention of health-care professionals, Dr. Schönthal said.
"I think the primary point would be to spread the message that physicians can tell their patients they have to avoid green tea and the green tea products."
Dr. Schönthal embarked on the study to determine how green tea might boost Velcade's effectiveness. In a study involving mice, however, the researchers quickly learned that molecules in the tea deactivate the drug.
The finding touches on an important issue that is creating concern among a growing number of health experts. Millions of Canadians take herbs, vitamins and supplements to improve their health. But in many cases, little research has been done to determine if natural health products cause potentially dangerous side effects when mixed with certain drugs, and even food products.
Consumers shouldn't assume natural remedies are safe, and should become educated about the potential risks of such products, as well as the benefits, according to Bruce Holub, professor of nutritional sciences at the University of Guelph.
"I don't think we can ever be 100 per cent confident with any of these [natural products]."
Dr. Schönthal said herbal remedies may have health benefits, but more research must be done to better understand potential risks as well.
"It's quite likely that most of these herbs are probably harmless and in the case of green tea, under certain circumstances, they're probably beneficial, but in this particular case ... it's clear that green tea is harmful."