Green tea is often the refuge of heart disease patients, who are told it is a much healthier source of caffeine than tea and coffee.
However, Japanese researchers have discovered it may block the effects of nadolol, a commonly prescribed pill to reduce high blood pressure.
They tested people who drank green tea as well as taking the beta-blockers, and found that they had lower circulating blood levels of the drug, the BBC reported.
SEE ALSO:
Heart Condition Affects More Than A Million In UK
People With Diabetes More Likely To Be Diagnosed With Heart Failure
The Mail Online added: "Drinking green tea cut the amount of drug that made its way into the bloodstream by three-quarters. It also meant that the drug, which is also taken to treat angina and irregular heartbeats and to prevent migraines, was less effective at lowering blood pressure.
"It is thought that plant chemicals in green tea stop nadolol from being ferried from the gut into the bloodstream, where it can get to work on reducing blood pressure."
The study was published in the Clinical Pharmacology & Therapeutics journal.
At present, the leaflet accompanying use of the tablets say that certain herbal remedies can interfere, but it doesn't include green tea.
The BBC added: "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.'"
Story continues below the slideshow:
So why is it recommended in the first place? Green tea is less processed than other tea, so it has a high level of antioxidants, which is good for the immune system.
However, simply because green tea may interact with blood pressure tablets such as nadolol doesn't mean it isn't a healthy addition to a person's diet. It has also been praised for having anti-cancer properties due to its antioxidant levels.
And - for older people - it can play a very beneficial role in bone strength.
"Osteoporosis is a major public health concern but new research suggests that polyphenols in green tea may help improve bone quality and strength through many proposed mechanisms," said a report in the American Journal of Clinical nutrition.
"In fact, one study found that tea drinking was associated with a 30% reduced risk in hip fractures among men and women over 50 years old. In a study of 150 postmenopausal women, researchers reported that 500 mg green tea extract (equivalent to 4-6 cups of green tea daily), alone or in combination with Tai Chi, improved markers for bone formation, reduced markers of inflammation and increased muscle strength in study participants. Numerous other studies have found that green tea flavanols provide a restorative effect to bone remodeling to help maintain bone density and slow bone loss."
If you are concerned about any of the above, talk to your GP.