NEW YORK - Go ahead. Have that second cup of coffee - or maybe even a third.

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Despite previous concerns, downing lots of coffee doesn't seem to increase the risk of high blood pressure, says a US study.

High blood pressure, or hypertension, has been linked to heart disease, stroke and a shorter life expectancy, and some scientists have suggested that coffee might fuel the problem.  

But according to a report in The American Journal of Clinical Nutrition that pooled data from six previous studies, covering 170,000 people, this did not appear to be the case.

"The results suggest that habitual coffee consumption of more than 3 cups per day was not associated with an increased risk of hypertension compared with less than one cup per day," wrote Liwei Chen, from the Louisiana State University School of Public Health in New Orleans, who worked on the study.

But Chen added that more data would be needed in order to draw a firm conclusion, noting that the report "is not saying there's no risk" to drinking lots of java.

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.

The studies 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 eventually developed high blood pressure, with the chance of being diagnosed with the condition no different for people who said they chugged more than five cups of coffee a day and those who drank very little.

"I don't think of coffee as a risk factor for high blood pressure," said Lawrence Krakoff, who studies high blood pressure at the Mount Sinai Medical Center in New York and wasn't involved in the new study.

But "if people are drinking 12 cups a day and aren't sleeping, I assume that's an important issue."

The relationship between coffee drinking and blood pressure is complicated by the possibility that it doesn't work the same way for everyone, Chen said.

"People with a different genetic background may react to coffee differently," she said.  

"For some people maybe it's safe to drink a lot of coffee, but not for other people."