Metformin is, under brand name Glucophage, a well-established drug to treat type 2 diabetes. Oncology also has an interest in metformin as observational studies suggest a survival benefit for people with hormone dependent cancers, including ovarian, breast and endometrial cancers. But even “simple” drugs with a long history of clinical use can have complex and unexpected side effects. In this regard a recent study reported by The Scientist caught our eye. A cohort study based on data from Denmark’s federal registry of healthcare shows that boys (but not girls) whose fathers took the type 2 diabetes drug metformin three months before their conception were more likely to have a birth defect. Confirmation and a mechanism are needed but the finding is thought-provoking – and definitively worth further study.