My wife was recently on metformin for a while. According to her, even if metformin does prevent aging in humans (which is not at all certain), it would not be worth the side effects.
If she was prescribed 1g tablets, for example, have her ask for a new script for twice as many 500mg tablets. (Those 1g are a pain to split.)
In my wife's case, it was nausea and hypoglycemia.