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Long-term use of a common class of high-blood-pressure drugs called calcium-channel blockers may be associated with higher risk of breast cancer in older women, a new study by the Fred Hutchinson Cancer Research Center has found.

Researchers interviewed 2,763 women in the Puget Sound area — two-thirds of whom had breast cancer — about the drugs they took for high blood pressure, or hypertension. Only calcium-channel blockers were found to be associated with breast cancer.

The proportion of women who got breast cancer was double in the group that had been taking those drugs for 10 or more years. Common calcium-channel blockers include amlodipine (Norvasc) and diltiazem (Cardizem).

The results were published online Monday by JAMA Internal Medicine.

Despite the correlation, researchers said the study cannot prove a causal link between the drugs and breast cancer because it was not a randomized trial, and because past studies have found conflicting results. More research is needed to establish any definitive link.

“These are some intriguing findings, but we do not think they should change current clinical practice in any way,” said Christopher Li, the study’s lead researcher and an epidemiologist at the Hutch.

Doctors warn that the benefit of taking medication for high blood pressure outweighs the possible risks raised in the study. High blood pressure can damage the blood vessels, heart and kidneys, which can lead to stroke, coronary heart disease and kidney problems.

In the current study, interviewers from the Hutch visited women 55 to 74 years old in King, Snohomish and Pierce counties between 2000 and 2008. They asked the women about their medical history, and in many cases transcribed prescription information straight from pill bottles, cutting down on recall errors.

That the study included only Puget Sound-area women may limit its wider applicability, experts say.

“The demographics here don’t reflect the multicultural America,” said Keith Ferdinand, a cardiologist and board member of the American Society of Hypertension, who was not involved in the study. He called the results “provocative,” but said plenty of associations in observational studies such as this one don’t pan out in randomized trials.

On the whole, a larger proportion of women in the study were white and wealthy compared to the general population.

Race can be a factor in choosing blood-pressure medication: Another class of drugs called beta blockers tend to be less effective when prescribed alone in African-Americans, who are especially prone to high blood pressure. There are several different types of hypertension drugs that work through independent biological mechanisms, and patients are given a combination tailored to their situation.

Calcium-channel blockers are the fourth most commonly prescribed class of hypertension drugs. Generic versions are available, and nearly 98 million prescriptions for these drugs were filled in 2010. Calcium-channel blockers also generally have mild side effects.

The drugs lower blood pressure by relaxing muscles in the blood vessels and heart. However, calcium widely affects all cells in the body, and the study’s author Li suggested calcium-channel blockers may increase cancer risk because they prevent programmed cell death of abnormal cells. This has not yet been proven by research.

Meanwhile, high blood pressure does cause cardiovascular disease, which is the leading cause of disease for women, over cancer.

“Managing hypertension is very important because of the negative health effects,” Li said. “That is much more important at this point than this very preliminary association.”

Sarah Zhang: 206-464-2195 or On twitter @sarahzhang