Drinking two or more cups of coffee daily is linked to double the risk of death from heart disease if you have severe high blood pressure, a new study from Japan suggests.

Previous studies have found that coffee consumption can reduce the risk of high blood pressure and death, but those studies didn’t differentiate among people with different levels of high blood pressure, according to study author Masayuki Teramoto, MD, MPH, of The University of California in San Francisco.

“Caffeine’s harmful effects may outweigh its protective effects in persons with severe hypertension and increase the risk of mortality,” Teramoto says. The study also looked at green tea consumption and found no link with heart disease death at any blood pressure level. For the study, published in the Journal of the American Heart Association, researchers analyzed the blood pressure levels plus coffee and green tea consumption of 18,609 Japanese people ages 40 to 75. Sixty-five percent were women.

Coffee consumption was categorized as occasionally or none, less than one cup, one cup, and two or more cups per day. For green tea consumption, the categories were occasionally or none, less than one cup, one or two cups, three or four cups, five or six cups, and seven or more cups per day. A total of 842 heart disease deaths occurred during the follow-up period at about 19 years. Coffee consumption was related to increased heart risk only among those who started the study with severe hypertension. Compared with non-coffee drinkers, their risk of heart disease death doubled with two or more cups of coffee per day.

Green tea consumption was not associated with an increased risk of death from heart disease at any blood pressure level. Rather, frequent consumption was associated with lower total cholesterol level among those with severe hypertension. More research is needed in different populations to confirm the study results, Teramoto says.

The study is unique in that it looks separately at risk associated with coffee and tea intake across different ranges of blood pressure, says American Heart Association Nutrition Committee Chair Christopher Gardner, PhD. But the study’s many categories for blood pressure and coffee and tea consumption made for very small subgroups for analysis. For example, only 19 (2%) of the 842 deaths were in the severe hypertension group.