It’s a health warning Americans have heard for years: Only use antibiotics when necessary, or you might end up with an infection that is difficult to treat because it contains bacteria that are resistant to these drugs.

But there may also be other reasons to only take antibiotics when you really need them. A new study finds that older women taking antibiotics for two months or longer in late adulthood have a higher risk of death from any cause and a higher risk of death from cardiovascular disease compared to women who did not use antibiotics.

The study included 37,516 women ages 60 and older who were enrolled in the Nurses’ Health Study, a long-term observational study launched in 1976 to identify long-term effects of diet, exercise, medication use and other factors on women’s overall health and their risk for specific diseases. It is funded by the National Institutes of Health.

Dr. Lu Qi, HCA Regents Distinguished Chair and professor of epidemiology at Tulane University in New Orleans, who led the study, presented the findings Wednesday at the American Heart Association’s Epidemiology and Lifestyle conference in New Orleans.

Because this is an observational study, it can only show a correlation between antibiotic use and heart disease. It “doesn’t necessarily mean a causal link,” Qi said.

The women enrolled in the Nurses’ Health Study are regularly sent questionnaires about their health. In 2004, the women were asked how often they had used antibiotics, on a scale from not at all to two or more months, during two separate time periods: between ages 40 to 59 and at age 60 or over. At that time, none of the 35,516 women Qi and his team studied had been diagnosed with heart disease or cancer.

By mid-2014, 4,535 women had died, 600 from cardiovascular-related causes and 1,179 from cancer. Analyses showed women who had taken antibiotics for two months or more were 19 percent more likely to die of any disease and 57 percent more likely to die from cardiovascular disease than those who had not taken any antibiotics.

The relationship between antibiotic use and increased risk of death, including death related to cardiovascular disease, was more evident among women who also had used antibiotics for two months or more in middle age than among those who only used antibiotics at age 60 or over.

The analysis took into account other factors that might increase heart disease risk, such as dietary factors, high blood pressure, diabetes and use of other medications.

Previous studies have found antibiotics can cause changes to gut microbiota, the microorganisms that live in the digestive tract. Researchers are currently investigating whether microbiota change in ways that increase a person’s risk of developing or dying from heart disease, cancer or other chronic illnesses.

Qi said his team looked for but did not find a relationship between antibiotic use and the risk of dying of any type of cancer.

“I think the study is fascinating,” said Dr. Nieca Goldberg, director of NYU Langone Medical Center’s Joan H. Tisch Center for Women’s Health and a clinical associate professor of medicine at NYU’s School of Medicine. She said this is the first time she’s seen a study look at the relationship between antibiotic use and heart disease.

However, Goldberg said more research is needed to determine if there is a causal link and what triggers it.

“We don’t want people who really need antibiotics being afraid to take them,” she said.

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