[sithadmin]

Not sure it qualifies as “nasty”, but performing does seem to somewhat impede expected benefits from exercise: https://peterattiamd.com/metformin-and-exercise-deja-vu-all-...

It also seems to upregulate cortisol in some cases, which is undesirable: https://pubmed.ncbi.nlm.nih.gov/27459533/

[prepend]

These definitely don’t seem very nasty to me. I suppose it’s good to be aware of these effects, but don’t imagine them affecting most people’s decision to take it unless they are bodybuilders or something.