Metformin, a first-line diabetes drug used for decades, may boost the risk of birth defects in the offspring of men who took it during sperm development, according to a large Danish study. Sons born to those men were more than three times as likely to have a genital birth defect as unexposed babies, according to the paper, published in the Annals of Internal Medicine today.

The genital defects, such as hypospadias, when the urethra does not exit from the tip of the penis, were relatively rare, occurring in 0.9% of all babies whose biological fathers took metformin in the 3 months before conception. But epidemiologists say the findings are important because tens of millions of people worldwide take metformin, chiefly for type 2 diabetes.

“When I saw the paper … I thought: ‘Yup, this is gonna go viral,’” says Germaine Buck Louis, a reproductive epidemiologist at George Mason University who wrote an editorial accompanying the report. “[Metformin] is widely used even by young men because of the obesity issue that we have. So that is potentially a huge source of exposure for the next generation.”

However, Buck Louis and every other scientist interviewed for this article stressed that the paper’s findings are preliminary and observational and need to be corroborated; they add that factors besides metformin may have influenced the findings. The scientists cautioned men with diabetes ag