Using a type of blood pressure drug for more than a decade may raise breast-cancer risk, a study found, the first potential link between long-term use of the popular medicines and the most common malignancy among women.
Women who took blood pressure drugs called calcium-channel blockers, like Pfizer's Norvasc, for 10 years or longer had a 2.5 times higher risk of developing breast cancer than those who never took the medicine or who used other blood pressure treatments, according to research published today in JAMA Internal Medicine.
The study is the first to find that long-term use of calcium-channel blockers is associated with a higher risk of breast cancer, the authors said. More studies are needed to replicate the findings and to better understand why the class of medicines may raise risk of the disease, said Christopher Li, the lead study author.
"While this is an intriguing finding, much more work still needs to be done," Li, head of the Translational Research Program at the Fred Hutchinson Cancer Research Center in Seattle, said Monday in an email. "While we appreciate that these results may cause concern for women currently using these medications, we do not think that they should change current clinical practice because they require confirmation and because of the importance of managing hypertension effectively."
Blood pressure drugs are the most commonly prescribed medicines in the U.S., the authors said. More than 678 million blood pressure prescriptions were filled in 2010, about 98 million of which were for calcium-channel blockers. Norvasc, which is available as a generic in the U.S., generated about $1.35 billion in 2012 sales for New York-based Pfizer mostly on sales outside the U.S.
Calcium-channel blockers lower blood pressure by relaxing blood vessels and preventing calcium from entering cells. Other treatments for the condition include beta blockers, which make the heart beat slower and with less force, dropping blood pressure; diuretics, which drain the body of excess fluid that may increase pressure; and angiotensin-converting enzyme, or ACE, inhibitors that prevent the body from producing angiotensin, which can narrow blood vessels.
The study included 1,960 women with breast cancer and 891 women without the disease. They were interviewed to determine which blood pressure treatments they used and for how long.
The data show the current use of calcium-channel blockers for 10 years or more increased breast cancer risk by about 2.5 times. Other blood pressure treatments used long term didn't have similar risks, the authors wrote.
Li said it's unclear why calcium-channel blockers may raise a woman's chance of developing breast cancer. He said the findings only apply to older women. Also, too few women stopped using the drugs so researchers were unable to evaluate breast cancer risk in those who were no longer using the medications.
"These findings raise a hypothesis," said Patricia Coogan, a research professor of epidemiology at the Slone Epidemiology Center at Boston University, who wrote an accompanying editorial. "It does not prove that using calcium- channel blockers for 10 or more years will raise your risk of breast cancer. More observational studies need to look in more populations. If this finding is replicated, then there needs to be a serious discussion about long-term use of calcium-channel blockers."
More than 232,000 women will be diagnosed with breast cancer in the U.S. this year and more than 39,600 will die of the disease, according to the National Cancer Institute, which funded the study.