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Researchers have found that the drug metformin, which is commonly used to treat diabetes, may cause genital birth defects such as undescended testicles and urethral problems in the male children of men who take the medication.

The use of Metformin appeared to affect sperm that developed during a critical time before a male child was conceived. 

Female children were not affected.

Studies have previously linked diabetes to fertility problems in men. But, according to the researchers, this latest study is the first to show that these problems can result from treatment rather than the disease itself. Their findings appear in the March 28 issue of Annals of Internal Medicine.

Because its the first to suggest a father’s use of Metformin may be linked to birth defects in his children, Michael Eisenberg, MD, Director of Male Reproductive Medicine and Surgery at Stanford University School of Medicine, said it would be too “early” to make any changes based on the data.

For this particular study, Eisenberg and his colleagues analyzed records in a database of all 1.25 million births in Denmark between 1997 and 2016. The database included information on birth defects and parents’ drug prescriptions.

Children were considered “exposed” to a diabetes drug if a father had filled one or more prescriptions for the medications within three months before conception when the fertilizing sperm would have been produced. The final analysis included over one million children, of whom about 7,000 were exposed to diabetes drugs via the father, and about 36,000 had one or more major birth defects.

According to the researchers, among nearly 1,500 male children whose fathers had taken Metformin, there were 3.4 times as many major birth defects of the genitals and urinary tract. However, the researchers did not find significant associations between birth defects and the use of insulin or sulfonylurea-based drugs.

Researchers didn’t find a risk of birth defects for the children of men who were prescribed Metformin in the year before or after their fertilizing sperm developed. They also found no increased risk for the siblings of the boys with birth defects who weren’t considered to have been exposed to the medication.

Germaine Buck Louis, Ph.D., a reproductive and perinatal epidemiologist, wrote in an accompanying editorial, “Given the prevalence of metformin use as first-line therapy for type 2 diabetes, corroboration of these findings is urgently needed.”

If other studies confirm the findings, doctors may begin discussing the possibility with patients.

Eisenberg added that eating a nutritious diet, exercising, and maintaining a healthy body weight “can improve a man’s health and likely his fertility as well.”

In the meantime, keep taking any medication as directed by your doctor. Overall, Metformin has been shown to be very safe and effective in its primary use with diabetes. If you think it’s possible that this study reflects an issue you could be having, bring the details to your doctor and discuss it with them.