Go ahead, ladies, ask for that extra shot of espresso in your morning latte. The caffeine jolt might spike your blood pressure for a short time—until lunch, perhaps—but it won't have a lasting effect, according to a study out this week in the Journal of the American Medical Association. The study followed more than 155,000 women for 12 years, asking them every couple of years about their consumption of caffeinated beverages and whether they had been diagnosed with high blood pressure by a doctor. Researchers found no association between total caffeine intake and high blood pressure.

However, other ingredients in caffeinated beverages might have an effect on heart health. When the researchers looked at the types of drinks women chose that contained caffeine, they found that one or more cans of soda a day boosted the chances of developing high blood pressure, while more than three cups of coffee a day actually reduced it. It's not clear why soda acts differently from coffee, says lead author Wolfgang Winkelmayer, an internist and nephrologist at Brigham and Women's Hospital in Boston. But he points out that coffee contains antioxidants that might protect women's hearts.

Or perhaps the differences have to do with the people, not the drinks, theorizes Daniel Rader, a cardiologist at the University of Pennsylvania School of Medicine not involved in the study. People who habitually drink coffee might be very different people from those who usually drink soda, he says, and "it's almost impossible to control for all the possible variables."

In any case, nonimbibers needn't run to the nearest coffeehouse. Women who drank more than three cups a day were only 7 to 9 percent less likely to have high blood pressure than those who drank none, not enough to justify using the beverage as a heart-healthy strategy, Winkelmayer says.

Previous studies that have found that caffeine boosts blood pressure for a very short time have led to speculation that there must be a long-term effect as well. Now that it appears that that's not the case in women, Winkelmayer's team next plans to look at men.

Find out more:

The American Heart Association has information about caffeine

So does the U.S. National Library of Medicine