Participants were from 40 to 79 years old when they joined the program between 1988 and 1990, and were followed until 2009. They self-reported their daily coffee and tea intake during the study period.

The researchers split participants into five categories of blood pressure—two normal and three grades of high blood pressure.

Less than 130/85 was considered optimal/normal, with 130-139/85-89 considered to be high normal.

Study data showed two or more cups of coffee per day was associated with double the risk of cardiovascular disease death in people with blood pressure of 160/100mmHg or higher compared to those who didn’t drink any coffee.

Drinking one daily cup of coffee or any amount of green tea wasn’t associated with increased risk of death from cardiovascular disease for any blood pressure categories.

People who drank coffee more frequently also tended to be younger, be current drinkers, eat fewer vegetables, and have higher total cholesterol levels and lower systolic blood pressure regardless of their assigned category.

Co-author Masayuki Teramoto, M.D., Osaka University Graduate School of Medicine in Japan and department of epidemiology and biostatistics at the University of California, San Francisco, told The Epoch Times that these findings don’t necessarily contradict previous research finding that coffee reduced the risk of heart disease.

Teramoto said that this study looked at whether the known protective effect of coffee also applies to individuals with different degrees of hypertension, while also examining the effects of green tea in the same population.

“To the best of our knowledge, this is the first study to find a positive association between heavy coffee consumption and cardiovascular disease mortality among people with severe hypertension,” said Teramoto.

He believes this is because people with high blood pressure are more susceptible to the effects of caffeine.

“As it is a retrospective analysis with patients in that cohort until 2009,” he said, as too much time passed since data was gathered.

He described the study data as being “all over the place,” regarding coffee consumption, and risk of cardiovascular events. Also, the data was self-reported which could have affected its accuracy.

“Is there a direct relationship between coffee consumption and the risk of higher cardiac events, or is it the fact that their blood pressures are too high, where they would have a higher risk of cardiac events anyway?” Gudimetla asked.

Self-reported data only provides information about the potential association of cause-and-effect, not a direct link between cause-and-effect.

“The reliability and validity of self-reported studies can be very flawed,” he said. “As the recall of data can be potentially inaccurate, and subjects tend to respond to questions in either extreme, and not necessarily in between.”

“However, in these studies, there was an association that most of the individuals enrolled were smokers and/or sedentary, which probably played a bigger part in their adverse cardiovascular outcomes than coffee consumption,” said Gudimetla.

“Moderate coffee consumption, i.e., 2 to 5 cups a day,” he explained, “has not been shown to be associated with an increase in cardiovascular events but has been shown to potentially decrease the risk of cardiovascular disease.”

Although there is one particular situation where caffeine might do harm.