Men can take the widely prescribed diabetes drug metformin without fear of causing birth defects in their children, according to results of a large study published on Wednesday.

Tracking more than 3 million pregnancies in Norway and Taiwan, researchers found no association between birth defects and use of metformin by fathers during the three months before conception, which is the period of sperm development.

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Metformin, a relatively inexpensive generic medicine, is typically the first drug prescribed for Type 2 diabetes, by far the more prevalent form of the disease.

A 2022 study from Denmark had found that metformin was associated with a 1.4 times greater risk of birth defects in boys whose fathers were taking the drug. Studies conducted since have not confirmed that association.

In June, a pair of studies published in Annals of Internal Medicine suggested that neither maternal nor paternal metformin use increases the risk of congenital malformations.

The use of data from two different populations in Norway and Taiwan strengthens the new study's results, Dr. Allan Pacey, an expert in male reproductive health issues at the University of Manchester in the UK, said in a statement.

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The new finding that no link exists between a father's metformin use and birth defects "makes sense because there was never any plausible biological mechanism which might have led us to this conclusion," Pacey said.

“I think the reasons that this new paper finds no link, compared to the 2022 paper which did, is simply down to data quality,” added Pacey, who was not involved in either study.

“Both analyzes were very well conducted, but in this paper... the authors were able to take into account many more confounding factors,” he added.

The new study was published The BMJ.