In Japan, 5.5bn bottles of green tea were consumed last year. Yet in Britain, green tea is drunk by few. Which is a pity, because it is probably the healthiest choice. Like black tea, green tea is made from the leaves of the tea plant, Camellia sinensis. The difference is essentially that, for black tea, the leaves are fermented, while for green tea they are not. Green tea therefore contains plenty more chemicals called polyphenols. These are powerful antioxidants with exotic names, such as catechins, epicatechin, catechins gallate and epigallocatechin gallate. It is these ingredients that may make green tea good for our health.

Years ago, epidemiologists noted that cancer rates in populations that consume green tea were lower than expected. We should not get too excited about such findings. For instance, tea drinkers could also be avoiding things that cause cancer or have a lifestyle that protects them. But encouraging results about green tea kept coming in and eventually formed a compelling body of evidence. The curiosity snowballed and, currently, research into the health aspects of green tea is buoyant.

Studies in test tubes show that the ingredients of green tea inhibit tumour growth and cause the death of cancer cells. In animal experiments, green tea impedes the development of chemically induced cancers. Some green tea ingredients seem to enhance the effect of anticancer drugs. Other compounds protect our organs against the damage that cancer drugs can have, for instance, on the heart. Taken alongside chemotherapy, green tea could maximise the benefits of such drugs and minimise their risks.

These effects may be valuable for a range of cancers. Importantly, they are supported not just by one or two investigations but by dozens of studies from around the world.

But the proof of the pudding is in the eating. Do we have data from clinical trials, or is all this based on lab experiments? So far few such studies have been completed. A rare exception is a prospective investigation from China of 254 women with ovarian cancer. While 78% of the green tea drinkers survived for longer than three years, the figure was only 48% for the abstainers. The authors of this study therefore believe that "increasing the consumption of green tea ... may enhance epithelial ovarian cancer survival". Another analysis found similar effects for sufferers of prostate cancer.

Antioxidants in green tea are not only important for cancer; they might also play a role in cardiovascular disease. Regular green tea consumption normalises lipid metabolism, reduces blood pressure, slightly lowers body weight, stabilises glucose metabolism in diabetes patients, and might even neutralise some effects of smoking. Collectively these effects are likely to amount to a significant protection from heart disease, stroke and other cardiovascular problems.

However, clinical trials are again scarce. A Japanese team observed 203 patients who underwent a coronary angioplasty. Of these, 109 had coronary artery disease while the rest had normal coronaries. Patients with normal coronary arteries consumed significantly more green tea compared to those who had diseased coronary arteries. The authors were optimistic: "The more green tea patients consume, the less likely they are to have coronary artery disease."

Before you rush out to buy a car load of green tea, a word of caution. All these findings are encouraging but, to be sure, we really need the results of clinical trials. These will take a while to come through. The good news is that green teais delicious and refreshing. The bad news is that to match the dose used in the research studies, you need to drink up to 12 cups a day.

· Edzard Ernst is professor of complementary medicine at the Peninsula Medical School at the universities of Exeter and Plymouth