A new study published in the Journal of Clinical Endocrinology & Metabolism finds that people who develop type-2 diabetes used significantly more antibiotics—even 15 years before they were diagnosed—than people without the disease.

The researchers used data from citizens of Denmark, since the country has a single-payer national health insurance system with detailed health and pharmacy records. Using national health registries, the researchers looked at 170,504 people who had type-2 diabetes and tracked their prescriptions for antibiotics. They matched those people with 1.3 million other Danish citizens of the same age and sex who did not have diabetes and compared data from the two groups.

People with type-2 diabetes were more likely to receive more courses of antibiotics (0.8 prescriptions per year) than those who didn’t have the disease (0.5 prescriptions per year). An increased use of antibiotics was detected up to 15 years before people with type-2 diabetes were even diagnosed; an increased usage was also found after diagnosis.

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The study wasn’t designed to be able to determine a cause, but the authors point out two interpretations of the results. One is that people who go on to develop type-2 diabetes are more vulnerable to infections years before they’re diagnosed, so it would make sense for them to take more antibiotics. The other theory is that antibiotics raise the risk of type-2 diabetes.

This second theory is derived largely from research in rodents, and it’s the one that compels Dr. Martin Blaser, professor of medicine and microbiology at New York University Langone Medical Center and a longtime researcher of the effects of antibiotics. (Blaser was not involved with this study but is familiar with the research.)

“When you take antibiotics,” he says, “you change the composition of the microbiota”—your personal collection of bacteria in the body that communicates with human cells. Blaser has a hypothesis for how this might work in children who develop type-1 diabetes: the change in composition from taking antibiotics also alters metabolism, possibly making people more likely to become diabetic. “This work coming out that antibiotics might be affecting adults in a similar fashion is a little surprising to me, but in no way shocking,” he says. “I thought by the time you’re adult things are pretty much fixed, but maybe they’re not.”

The results of the study are not conclusive. But Blaser says they add to the body of research about the potential risks of antibiotics. “It’s evidence consistent with the idea that antibiotics have cost—not just monetary cost, but a biological cost in terms of potentially causing long-term effects,” he says. “As we’re studying it more and more, it suggests that things may bounce back, but it may not be the same normal, and it may predispose to other diseases—including important diseases, common diseases, like type-2 diabetes.”

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Write to Mandy Oaklander at mandy.oaklander@time.com.