NEW YORK (Reuters Health) - Women taking a common type of blood pressure drug for a decade or more are over twice as likely to be diagnosed with breast cancer as those not on the medication, a new study suggests.
Researchers said the findings are not definitive enough to drive changes in how so-called calcium channel blockers are prescribed, and would first have to be replicated in other groups of women.
"We don't really think these results should change any current clinical practice," Dr. Christopher Li, who led the study at the Fred Hutchinson Cancer Research Center in Seattle, said.
"The most important thing is to have people manage their hypertension the best they can," he told Reuters Health, adding that people with high blood pressure often have to try multiple drugs at different doses to find the treatment regimen that works best for them.
Close to 98 million prescriptions were filled for calcium channel blockers in the U.S. in 2010, the researchers said - and once prescribed, people often take them for the rest of their lives.
Those drugs include amlodipine, marketed as Norvasc, and nicardipine, marketed as Cardene, among many others.
Li and his colleagues looked specifically at the two most common types of invasive breast cancer, ductal and lobular - distinguished by whether the cancer starts in a milk duct or milk-producing gland.
Their study included 905 women age 55 to 74 who were diagnosed with ductal breast cancer between 2000 and 2008, 1,055 women diagnosed with lobular breast cancer and 891 women without cancer who served as a comparison group.
Close to 40 percent of participants in each group took medication for high blood pressure.
Current users of calcium channel blockers included 85 women with ductal breast cancer, 91 with lobular breast cancer and 70 without cancer.
Twenty-five women with ductal breast cancer and 26 with lobular breast cancer had been using the drugs for 10 years or longer, compared to 11 cancer-free women. That worked out to a 2.4- to 2.6-fold higher risk of cancer with long-term use of calcium channel blockers.
However, no other hypertension drugs, including diuretics and beta blockers, were tied to breast cancer, the researchers wrote Monday in JAMA Internal Medicine.
Li said it's possible that calcium channel blockers interfere with a natural self-destruct mechanism in sick cells.
"But that's very speculative," he added.
The study raises a hypothesis for researchers to test in the future, said Patricia Coogan, an epidemiologist from Boston University who wrote a commentary published with the new report. The drugs have now been around long enough, she added, that plenty of women have taken them for a decade or more.
"If other large studies … start finding the same thing, then a serious discussion needs to take place," Coogan told Reuters Health.
But for now, she agreed with Li that women who get their blood pressure under control using calcium channel blockers shouldn't stop taking them, or be too concerned about any extra risk of breast cancer.
"A far more immediate health risk is uncontrolled high blood pressure, and that is the most important thing to have in mind," Coogan said.
SOURCE: bit.ly/MbBLbb JAMA Internal Medicine, online August 5, 2013.