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By Michelle Roberts Health editor, BBC News online
Green tea can weaken the effects of a commonly prescribed blood pressure pill,
experts warn.
Japanese researchers found the herbal drink blocks special cell transporters
that normally help the body absorb the beta-blocker medicine.
In tests, people who drank green tea alongside taking their tablets ended up
with lower circulating blood levels of the drug nadolol.
Experts say consumers need to be aware of this interaction.
Like other drugs, the patient leaflet accompanying nadolol tablets warns that
certain medicines, including herbal remedies, can interact with their action.
But it does not include green tea in this list.
Continue reading the main story
Green tea
Comes from Camellia sinensis, the same plant as other teas
However, it is produced in a slightly different way to produce its unique
flavour
The leaves are not given the opportunity to oxidise (react with oxygen in the
air) as with black tea, leaving the teas green in colour
Doctors already advise that certain fruit juices, including grapefruit, can
interfere with some common medications, including beta blockers.
The study in the journal Clinical Pharmacology & Therapeutics found nadolol's
lowering effect on blood pressure was blunted in the 10 volunteers who agreed
to drink green tea.
Follow-up tests in the laboratory revealed that green tea blocked a drug
transporter present in the lining of the human gut that helps move nadolol into
the cells.
Weakening effect
The scientists estimate that a couple of cups of green tea would be enough to
have this effect in humans.
It is not clear if other types of tea have a similar effect.
And they point out that green tea is also purported to have many health
benefits.
Green tea is less processed than other teas and, consequently, retains higher
concentrations of antioxidants.
Sotiris Antoniou, Royal Pharmaceutical Society spokesman and a consultant
pharmacist in cardiovascular medicine, suggested to blood pressure patients who
still want to drink green tea that leaving a four-hour gap between cups and
taking their medicines might get round the problem.
He said: "This has yet to be confirmed and is only extrapolated from our
experience with grapefruit for this type of interaction.
"What is clear as healthcare providers is that we need to ask patients about
their consumption of various fruits and supplements such as grapefruit and
green tea, and this needs to be documented in the clinical notes, and where
appropriate provide information on avoiding green tea or grapefruit, or better
where possible to prescribe an alternative drug that is not affected by the
consumption of green tea.
Mr Antoniou added: "For any individual concerned, they can go to their local
community pharmacy where they can clarify any potential interaction."