New research has indicated that green tea can make a common drug used to treat blood pressure less effective.

Researchers in Japan have discovered that this popular drink can inhibit the drug nadolol from being absorbed into the bloodstream, thereby affecting its ability to treat hypertension effectively.

The research found that people in the trial who drank green tea while on their course of blood pressure tablets ended up with lower circulating blood levels of nadolol. Green tea is said to confer many health benefits.

The researchers say people with blood pressure should be made aware of this possible interaction between green tea and the drug. They found that the blood-pressure-lowering effect of nadolol was hampered in 10 people in the study who drank green tea and who were taking the drug.

The leaves used in green tea are not given the opportunity to oxidise as the leaves of black tea do, and this may be linked to the discovery that green tea can hamper the effectiveness of nandolol.

The research found that green tea blocked a drug transporter in the lining of the stomach that helps transport nadolol into the blood cells.

Experts are advising people with blood pressure blood who want to to drink green tea to leave a four-hour gap between drinking it and taking nadolol.

The study was published in the journal Clinical Pharmacology & Therapeutics.