For aficionados and those who aren’t so much, green tea is not only a delicious kind of tea, but it also comes with vast health benefits ranging from lowering risk for stroke to preventing Alzheimer’s. But while it may be friendly to the body, it’s not friendly to medication. According to a preliminary study, the herbal tea blocks the high blood pressure drug nadolol from working.

The study, which was conducted by researchers from Fukushima Medical University in Japan, involved 10 participants who took a single, 30 milligram dose of nadolol after drinking either green tea or water three times a day for 14 days. After spending two weeks letting the drug and tea’s effects wear off, the participants drank the other drink at the same frequency, and then took the drug again. Both times, the researchers took blood and urine samples. Participants who drank green tea had 76 percent lower concentrations of nadolol in their blood and 80 percent lower concentrations in their urine, The L.A. Times reported.

To determine why this might have happened, the researchers conducted cell culture experiments. They found that catechins — a type of antioxidant — in the green tea inhibited cell transporters that normally help medications travel through the intestinal walls and into cells on the other side. The results of the study were published in the journal Clinical Pharmacology & Therapeutics.

Although the scientists suspect that only a cup or two could be enough to have this effect, Sotiris Antoniou, a spokesman for the Royal Pharmaceutical Society in the U.K. and pharmacist in cardiovascular medicine, told the BBC that a four-hour gap between drinking tea and taking nadolol might avoid the problem. “This has yet to be confirmed and is only extrapolated from our experience with grapefruit for this type of interaction,” he said. Grapefruit and its juice have also been shown to affect the strength of cholesterol and blood pressure medications, according to HealthDay.

“Individuals who take nadolol and also consume green tea should be aware of this potential interaction and discuss this with their physician,” Dr. Gregg Fonarow, a professor of cardiology at the University of California, Los Angeles, who wasn’t involved in the study, told HealthDay.