Women taking the heart drug digoxin have an increased risk of breast cancer, according to a study of more than 2 million Danes.

Digoxin, marketed as Lanoxin and Digitek, is used by people with heart failure or with abnormal heart beats. But it can also act like the female hormone estrogen in the body, leading researchers to wonder if it might up cancer risk the same way estrogen treatment does in older women.

About two percent of the women who took digoxin at any point during the new study eventually developed breast cancer.

Former users had the same risk as those who had never taken the drug, while those currently on it were about 40 percent more likely to get breast cancer.

That extra risk is “worth noting,” said Dr. Timothy Lash of Aarhus University Hospital in Denmark, a breast cancer expert who was not involved in the study. But he added that it’s less impressive when you consider how few women actually developed the disease.

In the U.S., about one in eight women develop breast cancer at some point, according to the American Cancer Society.

For the new study, researchers led by Dr. Robert Biggar of Statens Serum Institut in Copenhagen, Denmark, tapped into two different country-wide databases - one that includes all prescriptions filled in Denmark and another that contains all cancer cases.

About 100,000 women age 20 and older took digoxin at some point during the course of the study, which tracked women for an average of 12 years.

The chance of getting breast cancer was highest in women who were in their first year of taking digoxin. Then it dropped off, but slowly crept up again after 3 years or more on the drug.

Digoxin is often added on top of other heart drugs, such as beta blockers and statins. There was no extra breast cancer risk linked to those drugs, however.

GlaxoSmithKline, which markets digoxin, was not available for immediate comment on the study.

The study can’t prove that digoxin, also known as digitalis, actually causes breast cancer in some women. And even if it does, the heart benefits might still outweigh that risk, the researchers write in the Journal of Clinical Oncology.

So the findings don’t mean women should avoid taking digoxin, Lash told Reuters Health. Those who are on the drug “are receiving digoxin because they already have a fairly serious chronic disease,” he said.

“It might be worth talking with their doctor about the tradeoff, (but) maintaining heart health is an important consideration,” he said.

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By Genevra Pittman

NEW YORK