Green tea has become a popular health drink said to be rich in vitamins and healthy plant chemicals.
But now researchers have discovered it can be harmful to some cancer patients – as it blocks modern drug treatments.
Researchers said it made the drug, Velcade, "completely ineffective".
Cancer specialists said the finding was alarming as green tea was popular with cancer patients and was thought to help them cope with side-effects of treatment.
Velcade – or bortezomib – is a powerful modern drug developed to treat multiple myeloma – cancer of the bone marrow – and mantle cell lymphoma.
The findings emerged from laboratory studies intended to measure the benefits claimed for adding green tea to cancer treatment and reported in the journal Blood. The researchers say they are also finding instances where green tea does help patients.
Researcher Dr Axel Schonthal, of the Keck School of Medicine, University of Southern California, USA, said: "The most immediate conclusion from our study is the strong advice that patients undergoing cancer therapy with Velcade must avoid green tea, and in particular all of its concentrated products that are freely available from health food stores.
"Essentially, in addition to not being able to attack tumour cells, Velcade would be unable to cause side effects either.
"As a result, the patient would feel a lot better and conclude that the consumption of green tea helped cope with side effects—while in reality, Velcade simply wasn’t active in the first place."
Blood online February 3, 2009; DOI 10.1182/blood-2008-07-171389
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