Despite earlier concerns, downing lots of coffee doesn't seem to increase the risk of high blood pressure, according to a new report -- but the evidence isn't conclusive.
High blood pressure has been linked to heart disease, stroke, and a shorter life expectancy, and some scientists have suggested that coffee might fuel the problem.
The new report pools data from six previous studies that included more than 170,000 people in total. For each study, scientists surveyed the participants to find out how many cups of coffee they drank each day -- from less than one to more than five -- and then followed them for up to 33 years.
Just more than one in five participants eventually developed high blood pressure, according to the findings, which appear in The American Journal of Clinical Nutrition.
But the chance of being diagnosed with the condition was no different between people who said they chugged more than five cups of coffee per day and those who drank very little.
Still, the report "is not saying there's no risk" to drinking lots of java, Dr. Liwei Chen, who worked on the study, told Reuters Health.
Chen, from the Louisiana State University School of Public Health in New Orleans, said more data would be needed to draw a firm conclusion.
What's more, people who drank between one and three cups per day had a slightly higher risk of high blood pressure than those who drank less -- a result the researchers couldn't explain.
Dr. Lawrence Krakoff, who studies high blood pressure at the Mount Sinai Medical Center in New York, said that the question about coffee's effects "keeps popping up" among both his patients and fellow doctors.
But it has yet to be answered completely, said Krakoff, who was not involved in the new work.
"I don't think of coffee as a risk factor for" high blood pressure, he told Reuters Health. However, "If people are drinking 12 cups a day and aren't sleeping, I assume that that's an important issue."
Dr. Gary Curhan, who worked on one of the studies Chen and her colleagues looked at, agreed.
"There may be other adverse effects to (drinking) large amounts of caffeine," Curhan, of Brigham and Women's Hospital in Boston, told Reuters Health.
But based on the existing data, he said there is no reason to believe that drinking coffee would lead to high blood pressure.
Chen's team could not compare the effect of drinking caffeinated versus decaffeinated coffee, as some of the studies they analyzed had participants report both together or only asked about caffeinated coffee.
And the relationship between coffee drinking and blood pressure is further complicated by the possibility that it doesn't work the same way in everyone, she said.
"People with a different genetic background may react to coffee differently," Chen said. "For some people maybe it's safe to drink a lot of coffee, but not for other people."