A new study published in the Journal of the American Medical Association suggests that postmenopausal women who took calcium channel blockers for their blood pressure for more than 10 years were 2.4 to 2.6 times more likely to develop breast cancer compared to women who did not take any hypertension medications.
The study was observational and did not get into the reasons behind the increase, but experts said the findings may have major public health implications.
"While some studies have suggested a positive association between calcium channel blocker use and breast cancer risk, this is the first study to observe that long-term current use of calcium channel blockers in particular are associated with breast cancer risk," said the study.
Calcium channel blockers were the ninth most commonly prescribed drug in the United States in 2009, with over 90 million prescriptions filled, according to JAMA.
Examples include amlodipine, diltiazem, felodipine, isradipine, nicardipine, nifedipine, nisoldipine and verapamil.
The drugs prevent calcium from getting into the muscles of the heart and arteries, and may widen the blood vessels and slow the heart rate.
"Other antihypertensive medications - diuretics, beta-blockers and angiotensin II antagonists - were not associated with increased breast cancer risk," said the JAMA study.
Researchers examined breast cancer risk using a population of women age 55 to 74 in the western state of Washington. A total of 880 had invasive ductal breast cancer, 1,027 had invasive lobular breast cancer and 856 had no cancer and served as the control group.
They found that using calcium channel blockers (CCB) for 10 or more years was associated with 2.4 times higher odds of ductal breast cancer and 2.6 times higher odds of lobular breast cancer.
"The present study provides valid evidence supporting the hypothesis that long-term CCB use increases the risk of breast cancer. If true, the hypothesis has significant clinical and public health implications," said an accompanying editorial in JAMA by Patricia Coogan, senior epidemiologist at the Slone Epidemiology Center at Boston University.
"If the two- to three-fold increase in risk found in this study is confirmed, long-term CCB use would take its place as one of the major modifiable risk factors for breast cancer."
Breast cancer is the most common cancer in women worldwide. According to the National Cancer Institute, one in eight women born today will develop breast cancer in her lifetime.