Drug Profile

Background

The shrub Camellia sinensis is the source of the three major forms of tea in the world. (For these purposes, we are excluding “teas” made from rose hips, chamomile, etc., which are considered infusions and are not made from tea leaves.)

Green tea is prepared by drying and steaming the tea leaf, black tea is the same leaf but fermented, and oolong tea is partially fermented. Green tea is a popular drink in China and Japan, whereas in North America and Europe black tea is more popular. Approximately 3 billion kilograms of tea are produced and consumed annually.[1]

Green tea contains chemicals called polyphenols. These chemicals are antioxidants that block the action of free radicals, which can cause cellular and genetic damage. Antioxidants are found in many foods in addition to green tea, and certain antioxidants have been tested in large cancer prevention trials with no success. The polyphenols (flavonoids) in green tea are called catechins; these are converted into the theaflavins present in black tea during the fermentation process.

Claimed uses

Cancer treatment and prevention

Green tea is unique among the supplements reviewed in this chapter in that claims for its effectiveness rest primarily on epidemiological studies. (Most other supplements rely upon randomized clinical trials [often poorly conducted], anecdotes, or appeals to tradition or naturalness.) This alone should increase one’s level of skepticism about the claims made for green tea.

Although epidemiological studies are often the only way to study certain human risks (e.g., environmental or occupational exposures), no drug regulatory authority would approve a drug based on such evidence alone.

Some of the studies claiming a protective effect of green tea against a particular cancer are based on ecological studies, a particularly weak form of epidemiological study. These usually take the form of demonstrating that cancer rates are lower in areas with high consumption of green tea than in areas with lower consumption. These studies should be viewed with great caution, because such studies do not even include data on the green tea consumption of particular individuals with or without cancer.

The most common kind of study has been a case-control study, in which patients with and without cancer are asked about their prior consumption of green tea. However, patients with cancer may have diets that are different from patients without cancer in many different ways, not just with respect to green tea; they may even start taking green tea once the early symptoms of the cancer appear, producing a spurious result. Cancer patients are likely to recall their diets differently than patients without cancer. Indeed, all patients struggle to describe dietary patterns that stretch back decades.

A better study (although still not one that would satisfy a drug regulatory authority) would be one in which patients are asked about their diets on a periodic basis and then are followed to see if they develop cancer. This is called a prospective study. However, the results of such studies have been mixed.

For example, studies of tea consumption and stomach cancer have showed tea drinkers to be at greater risk,[2] at lower risk,[3] and at neither higher nor lower risk[4] compared to those consuming less or no tea. One study found that tea protects against pancreatic cancer,[5] but another study did not.[6] One study found that more frequent tea drinkers were at greater risk of lung cancer,[2] but another did not.[4]

These studies can be difficult to interpret because of perhaps unmeasured differences between the patients who do and do not develop cancer. Conceivably the difference between black and green teas is also important.

The best way to eliminate such biases is to conduct a randomized, controlled trial, because randomization makes it very likely that the two groups will be similar in most respects. No such study has been published.

The closest is an uncontrolled prospective study of 42 patients with a particular form of prostate cancer, all of whom were treated with green tea. Only one patient responded, and even this response was not sustained beyond two months. The authors concluded that “green tea, as administered in this study, does not merit further investigation in the treatment of patients with androgen independent prostate carcinoma.”[7]

The National Cancer Institute is conducting studies on the use of green tea for preventing skin cancer.[8]

Cholesterol lowering

We identified two randomized, placebo-controlled trials of green tea to lower cholesterol. In the first, a small trial, neither green tea nor black tea reduced the levels of total cholesterol, the components of total cholesterol or triglycerides.[9]

The second trial compared a theaflavin-enriched extract of green tea to a placebo.[10] This 12-week study showed a modest decrease in total cholesterol and LDL cholesterol (“bad cholesterol”), but no increase in HDL cholesterol (“good cholesterol”) or decrease in triglycerides compared to the placebo group.

Importantly, this was not a study of green tea per se, because the extract was enriched with flavonoids characteristic of black tea. Brewed green tea has a highly variable amount of various flavonoids.

Whether or not the differences claimed in this trial would be sustained, have any effect on disease outcomes, or be replicated in another study is unclear.

Interactions with other drugs

Interactions between green tea and anticoagulant drugs (e.g., warfarin [COUMADIN]) and other dietary supplements (including vitamin E and ginkgo biloba) have been reported. It is recommended that green tea use be discontinued at least 24 hours prior to surgery.[11]

An interaction between green tea and nadolol (a medication used to treat high blood pressure) has been reported. A study in healthy volunteers showed that drinking green tea resulted in a decrease in the blood concentration of nadolol. This suggests that green tea may decrease the drug’s effectiveness in patients with hypertension.[12]

Side effects

Because green tea contains 10–80 milligrams of caffeine per cup,[13] excessive amounts may lead to insomnia, rapid heartbeat, abnormal heart rhythms, nausea and vomiting.

Green tea has also been associated with gastrointestinal distress, loss of appetite, headache and vertigo (dizziness).[11]

Patients with kidney disease, overactive thyroid glands, anxiety, panic disorder, or cardiovascular disease should be cautious in their use of this supplement.[11]

Green tea–induced asthma due to inhalation has also been reported in 11 patients,[14],[15],[16] five of whom simultaneously developed worsened asthma after drinking green tea.[17]

According to the American Cancer Society, women who are pregnant or breast-feeding should restrict their intake of green tea.[18]

In 2009 an article was published looking at the relationship between green tea consumption and hepatotoxicity (liver toxicity). After a review of the case reports researched, the author concluded that:

Our analysis of the published case reports suggests a causal association between green tea and liver damage. … In a few cases, toxicity related to concomitant medications could also be involved.[19]

Conclusion

The health benefits of green tea remain unproven.