Metformin, one of the most commonly widely prescribed diabetes drugs, could increase the risk of major birth defects, according to a new study.

The study, published in the Annals of Internal Medicine, found that the risk of birth defects is high among offspring of male patients who were taking metformin ahead of the babies being conceived.

Researchers from the University of Southern Denmark and Stanford University in the United States have found metformin was associated with 1.4 times greater risk of birth defects in boys whose fathers were taking the drug.

However, the researchers, as well as independent experts, said there are several key limitations to the study. The investigators did not know whether the fathers took metformin as prescribed or if they had worse control of their diabetes, which could also increase the risk of birth defects.

Dr. Channa Jayasena of Imperial College London, who was not part of the study, called the findings “thought-provoking but inconclusive.”

He said, “Men with diabetes should not be dissuaded from taking metformin, but this is worth looking at more closely.”

Metformin is usually prescribed when patients fail to control their blood sugar through diet and physical activity. It improves how the body handles insulin. Globally, more than 120 million people have been prescribed metformin.

The researchers looked at 1,116,779 births in Denmark between 1997 and 2016. They found that more than 5% of babies born to men who had been taking metformin had birth defects, especially genital defects in boys.

The authors considered that babies were exposed to the diabetes drug if the father had filled at least one prescription during the three months before conception, when the fertilizing sperm were developing, per Reuters.

The researchers said more studies were needed but suggested that men who take metformin should consider switching to another drug when trying to conceive.

Study author prof. Maarten Wensink told Reuters, “If patients would like to switch to an alternative, they should contact their doctor,” adding that the best lifestyle interventions for people with diabetes include dietary changes and weight loss. “This could be an extra reason to put more priority on paternal health.”