Women who don't have high blood pressure may not need to worry that drinking coffee will increase their risk of developing hypertension, but the picture is less clear for drinking cola.

Experiments show caffeine increases blood pressure and heart rate in the short term, although the effect seems to weaken over time.

Dr. Wolfgang Winkelmayer of the Harvard School of Public Health and colleagues wanted to explore the long-term effect of caffeine intake, given how commonly it is consumed.

Over 12 years, the researchers tracked cases of hypertension diagnosed by a physician among 155,594 mostly white, female nurses aged 55 on average. The women answered questionnaires about their diet and health.

"We found strong evidence to refute speculation that coffee consumption is associated with an increased risk of hypertension in women, the study's authors wrote in the Nov. 9 issue of the Journal of the American Medical Association.

Over the course of the study, about 33,000 were diagnosed with high blood pressure, a condition that increases the risk for heart disease, stroke and kidney problems.

Winkelmayer said they were surprised to find that drinking cola, whether with sugar or sugarfree, was associated with greater risk of hypertension.

Women who drank at least four cans of sugared cola drinks daily had a 28 per cent to 44 per cent higher risk of high blood pressure, compared with women who drank few or none. The effect was slightly less among those who drank sugarfree cola.

The researchers speculate something other than caffeine may be responsible for the increased risk of hypertension from cola, after controlling for weight changes and sodium content of the drinks.

Winkelmayer said there is no reason to change cola consumption based on the results.

More research is needed to find the biological mechanism behind the association, and to see if it exists among other racial populations. The researchers are planning a similar caffeine study among male health professionals.

Current Canadian hypertension guidelines say there is no risk to coffee consumption among people who are not hypertensive.