Metformin may be linked to major birth defects in the offspring of male patients who take it ahead of conception, a Danish study shows.

The medication was associated with a 1.4 times greater risk of birth defects in boys whose fathers were taking the drug compared with those born to fathers who were not, the authors found.

In both groups, the mothers had no history of diabetes or hypertension, report researchers from the University of Southern Denmark and Stanford University, California, US.

Their study of almost 1.2 million births in Denmark from 1997 to 2016 showed that 5.2% of babies born to men who had been taking metformin had birth defects, particularly genital malformations in boys. Among the rest of the population, the rate was 3.3%.