Metformin is also sometimes used to induce ovulation in non-obese women of childbearing age with polycystic ovarian syndrome.[0] Luckily, this study seems to only show an effect if taken within the 90 day window before conception, so it seems to be doing something to the sperm, and not causing genetic changes. Also, of course, we'd want to see corroborating studies.
> it seems to be doing something to the sperm, and not causing genetic changes.
To be clear, spermatogenesis takes roughly 75-120 days, so this is consistent with metformin causing genetic changes to the sperm when or after they are formed, with those genetic changes being propagated through the germ line to children conceived by the sperm. But it's true that the (very preliminary) data does not suggest that this is causing permanent genetic changes in the father.
I think "sometimes" is a bit weak here. My second-hand understanding (from multiple women who have gone through this) is that it's routinely the very first treatment tried for women with PCOS who are having trouble conceiving.
[0] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4200666/