In an in vitro study, epicatechin gallate, a catechin constituent of green tea leaves, was shown to markedly lower the minimum inhibitory concentration of beta-lactam antibiotics against methicillin-resistant Staphylococcus aureus (MRSA) (Shiota et al. 1999). Epicathechin gallate may restore the effectiveness of beta-lactam antibiotics against MRSA, a major cause of nosocomial infections in hospitals that is resistant to many antibacterial agents including beta-lactams.

Theanine, a water-soluble amino acid from green tea leaves, has been shown to enhance the anti-tumor activity of doxorubicin in mice. In one study, the combination of theanine (10 mg/kg/day) with doxorubicin (2 mg/kg/day) reduced the tumor weight to 30% of the control level and inhibited hepatic metastasis of M5076 ovarian sarcoma (Sugiyama and Sadzuka 1999). In another study, administration of green tea powder (1 g/kg/day po for 4 days) to Ehrlich ascites carcinoma tumor-bearing mice enhanced the inhibitory effects of doxorubicin on tumor growth 2.5-fold (Sadzuka et al. 1998). Coadministration of doxorubicin with green tea increased the concentration of doxorubicin in the tumor, but not in normal tissue. Patients undergoing chemotherapy may benefit from drinking green tea, although clinical trials are needed to confirm these results.

In one study, a combination of green tea extract and tamoxifen significantly enhanced induction of apoptosis by (-)-epigallocatechin gallate (EGCG) in vitro (Suganuma et al. 1999). Drinking green tea may enhance the cancer-preventive activity of tamoxifen, requiring smaller doses with potentially fewer side effects.

Drinking large amounts of green tea may decrease the effectiveness of warfarin. Green tea contains vitamin K (approximately 0.03 microgram of vitamin K/100 g of brewed tea), which is known to inhibit the effects of warfarin (Taylor and Wilt 1999). In one case study, green tea consumption was reported to cause a fall in INR from a high of 3.79 to a low of 1.14 in a man treated with warfarin. He had begun drinking one-half to one gallon of green tea daily during his course of therapy with warfarin. The INR increased to 2.55 after discontinuation of the tea.

Sadzuka Y, Sugiyama T, Hirota S. Modulation of cancer chemotherapy by green tea. Clin Cancer Res. 1998;4:153-156.

Shiota S, Shimizu M, Mizushima T, Ito H, et al. Marked reduction in the minimum inhibitory concentration (MIC) of ß-lactams in methicillin-resistant Staphylococcus aureus produced by epicatechin gallate, an ingredient of green tea (Camellia sinensis). Biol. Pharm. Bull. 1999;22(12):1388-1390.

Suganuma M, Okabe S, Kai Y, Sueoka N, et al. Synergistic effects of (-)-epigallocatechin gallate with (-)-epicatechin, sulindac, or tamoxifen on cancer-preventive activity in the human lung cancer cell line PC-9. Cancer Res. 1999;59:44-47.

Sugiyama T, Sadzuka Y. Combination of theanine with doxorubicin inhibits hepatic metastasis of M5076 ovarian sarcoma. Clin Cancer Res. 1999;5:413-416.

Taylor JR, Wilt VM. Probable antagonism of warfarin by green tea. Ann Pharmacother. 1999;33:426-428.Tyler V. Herbs of Choice: The Therapeutic Use of Phytomedicinals. Binghamton, NY: Pharmaceutical Products Press; 1994.