Women 51-77 who take more than 400 mg a day of NSAIDS -- equal to say two ibuprofen -- had a 78 percent increased risk of developing high blood pressure within three years over those who didn't take the drug.

"If you're taking these over-the-counter medications at high dosages on a regular basis, make sure that you report it to your doctor and you're checking your blood pressure," said Dr. Christie Ballantyne, a cardiologist at the Methodist DeBakey Heart Center in Houston who had no role in the study.

While many popular over-the-counter painkillers have been linked before to high blood pressure, acetaminophen, sold as Tylenol, has generally been considered relatively free of such risk.

It is the only one that is not a non-steroidal anti-inflammatory drug or NSAID, a class of medications the federal government just required to carry stricter warning labels because of the risk for heart-related problems. Those include ibuprofen (sold as Advil and Motrin) and naproxen (sold as Aleve). Many had turned to those painkillers in the wake of problems with prescription drugs, such as Vioxx.

Aspirin Still Remains the Safest

However, the new study found that women taking Tylenol were about twice as likely to develop blood pressure problems. Risk also rose for women taking NSAIDS other than aspirin.

The research found that aspirin still remains the safest medicine for pain relief. It has long been known to reduce the risk of cardiovascular problems and was not included in the government's requirement for stricter labels for NSAIDs.

The study involved 5,123 women participating in the Nurses Health Study at Harvard Medical School and Brigham and Women's Hospital in Boston. None had had high blood pressure when it began.

Results were published online Monday in the American Heart Association journal Hypertension.

"It certainly sets the basis for more studies," said Dr. Stephanie Lawhorn, a cardiologist at St. Luke's Mid America Heart Institute in Kansas City. "Most of the time we think that things like acetaminophen are fairly safe drugs."

More Informed Choices

The study found that women ages 34-77 who took an average daily dose of more than 500 milligrams of acetaminophen -- one extra-strength Tylenol -- had about double the risk of developing high blood pressure within about three years.

Women 51-77 who take more than 400 mg a day of NSAIDS -- equal to say two ibuprofen -- had a 78 percent increased risk of developing high blood pressure over those who didn't take the drug. Those ages 34-53 had a 60 percent risk increase.

"We are by no means suggesting that women with chronic pain conditions not receive treatment for their pain," lead author Dr. John Phillip Forman, of Harvard Medical School and associate physician at Brigham and Women's Hospital in Boston, said in an e-mail. "By pointing out risks associated with these drugs, more informed choices can be made by women and their clinicians."

Previous research linking these drugs to blood pressure problems did not look at dose.

The results in this study held up even when researchers excluded women who were taking pills for headaches, something that could itself be a result of very high blood pressure, said Dr. Gary Curhan, another study author also of Harvard Medical School.

As for why aspirin didn't raise risk, it might be because "aspirin has a different effect on blood vessels than NSAIDS and acetaminophen have," said Dr. Daniel Jones, dean of the school of medicine at University of Mississippi Medical Center in Jackson.