Does metformin taken by men increase the risk of birth defects?

While the risk of birth defects with medications taken by women is fairly well studied, there is a dearth of literature regarding whether medications taken by men may increase the risk of birth defects in their offspring.

A recent study evaluated whether diabetes medications taken by men could affect risk of birth defects in their children. The study was a population-based registry of over 1.2 million births in Denmark between 1997-2016. They excluded any babies born to mothers with diabetes. They found a 40% higher risk of birth defects, particularly genital-urinary defects in boys, when their dads were on metformin during the window of time that the sperm was produced that conceived them (within 3 months before conception). They found no significant association between birth defects and dads’ use of insulin nor sulfonyureas (the sulfonylurea results were actually numerically similar, but were not statistically significant).

Now, we know that obesity and diabetes in men is associated with poorer reproductive health, due to factors including lower testosterone production and impaired sperm production. This study did not have data on obesity, blood sugar control, nor compliance with medication.

So, is the finding of increased birth defects with dads’ use of metformin related to metformin, or to some other factor? We don’t know the answer to this question, and more study of this question is urgently needed.

In the meantime, should men trying to conceive stop metformin? Again, we don’t have a clear answer on this. If the decision is made to stop metformin, it should be stopped 3 months prior to trying to conceive, as it takes about 3 months for sperm to develop. If metformin is stopped, alternative medication should be used to maintain good blood sugar control, as good diabetes control is also important for male reproductive health (and health overall!).

PS – Metformin use by mom has not been associated with an increased risk of birth defects, and is commonly used to treat diabetes in pregnancy.

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