Diabetes drug metformin linked to birth defects in baby boys, study finds

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A popular diabetes drug could be linked to major birth defects, according to researchers. 

In a study from Stanford University, the University of Southern Denmark and Copenhagen University, published Tuesday in the Annals of Internal Medicine, the authors wrote that the offspring of male patients who were taking metformin three months before conception were impacted. 

Sons born to those men were more than three times as likely to have a genital birth defect as unexposed babies, with metformin associated with a 1.4 times greater risk of birth defects in boys whose fathers were taking it versus those who were not.  

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The mothers in both groups had no history of diabetes or hypertension.

In addition, the fathers who used metformin during sperm development were less likely to have boys than girls compared with the general population.

Stanford said in a Monday release that its conclusions suggest that metformin affects men’s reproductive health in a way that can harm their sons, "though the mechanisms are still unclear."

To reach these results, the group used health-registry data from Denmark that tracked more than 1 million births from 1997 to 2016. 

The offspring were considered to be exposed if their father filled one or more prescriptions for a diabetes drug during the development of fertilizing sperm, and sex and frequencies of major birth defects were compared across drugs, times of exposure and siblings.

Of the more than 1.1 million babies included in the study, 3.3% had at least one major birth defect and 51.4% were boys. 

Among the 1,451 babies exposed to metformin, 49.4% were male and the rate of birth defects was 5.2%. 

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Stanford explained that the rates mean metformin use may account for two additional babies born with birth defects every year.

The college noted that the oral treatment is increasingly popular for Type 2 diabetes, but is known to have effects on the reproductive system.

The researchers also compared exposure to metformin with exposure to two other common diabetes drugs, insulin and sulfonylureas.

Exposure to insulin did not affect the rate of birth defects or sex ratio; exposure to sulfonylureas showed numbers trending in the same direction as metformin exposure, but they lacked a consistent pattern.

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Lastly, men who filled metformin prescriptions before or after the three-month period of sperm development did not have offspring with a higher incidence of birth defects and unexposed siblings did not have increased rates of birth defects.

Limitations to the study include that information on underlying disease status was limited, as well as whether fathers took the drug as prescribed. 

The authors called for further research that clarifies the causation of these defects.

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"I think that it’s a single study, so it’s hard to change clinical practice based on that," Dr. Michael Eisenberg, senior author and professor of urology at Stanford, said. "But, for somebody considering fatherhood, this study emphasizes the importance of a father’s health on the health of a child."

Ultimately, he said it will be important to uncover the biological mechanisms at work and to expand the study.

Data from Statista shows that 86 million prescriptions for metformin were written in the U.S. by 2019.