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Taking certain types of blood pressure drugs may increase women's risk of developing breast cancer, according to a new study.

A long-term study revealed that older women who take a class of antihypertensive drugs known as calcium-channel blockers are more likely to develop breast cancer.

Antihypertensive drugs are the most frequently prescribed type of drug in the United States. Researchers said that of the 678 million prescriptions filled in 2010, nearly 98 million were for calcium-channel blockers.

"Because hypertension is a chronic condition, most people with high blood pressure use antihypertensive drugs chronically and will often stay on the same regimen for long periods of time," lead researcher Dr. Christopher Li, M.D., Ph.D., said in a news release.

"Characterizing their potential associations with the most common cancer in women is an important clinical and public health issue, particularly with the increasing availability of alternative options to manage hypertension," he added.

Besides calcium-channel blockers, other classes of antihypertensive drugs include angiotensin-converting-enzyme (ACE) inhibitors, angiotensin-receptor blockers, beta blockers and diuretics. Combinations of drugs are also prescribed.

"Each drug has different potential benefits as well as side effects," Li said. "Choice of which regimen a patient is given depends on their tolerance of medication, other conditions, and whether their hypertension can be managed by a single drug or requires a combination of drugs."

The findings revealed that women currently taking calcium-channel blockers who have used tem for 10 years or longer had an approximately two and a half times higher risk of both invasive ductal and invasive lobular cancers compared to those who never used such calcium-channel blockers and compared to users of other forms of antihypertensives.

Invasive ductal carcinomas represent approximately 70 percent of all breast cancers, and invasive lobular carcinomas represent an about 20 percent.

Researchers also found that the use of other classes of antihypertensive drugs, including diuretics, beta blockers and angiotensin-receptor blockers, were not associated with an increased risk of breast cancer, even when used long term.

Researchers said more research is needed to understand the underlying biological mechanisms potentially responsible for the added cancer risk. Previous studies suggest that calcium-channel blockers may increase cancer risk because they inhibit programmed cell death, or apoptosis.

The findings are published in the JAMA Internal Medicine.