Metformin is fairly safe and toxicity doesn't occur until you reach doses that are many, many times higher than therapeutic doses [1] [2]. Not a doctor, not your doctor, talk to your GP before starting such a regimen (especially for healthy kidney and liver function). My prescription was a 3 minute convo after my annual physical ("its safe, my kidneys and liver are working, I want to live longer [3] [4], and I will sign a waiver I won't hold you liable if necessary"), and I would've just ordered it online if my doctor said no.
[1] https://care.diabetesjournals.org/content/35/4/731
[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5934269/