The assumption of many people is that it is safe for non diabetics because it is considered safe for millions of diabetics. Some disagree with that view. During my research, I haven’t found any human studies for the off label use of metformin as an anti-aging drug. There is a funded study which will start soon. Look at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5943638/
Life Extension has recommended metformin as an anti-aging drug since about 1995. Dr. Sandra Kaufmann, MD, recommends it for non-diabetics in her book, The Kaufmann Protocol. David Sinclair (Harvard Antiaging researcher and author of the book Lifespan) who has tested metformin in mice studies, personally uses metformin as a non-diabetic (but it seems he doesn’t take it everyday). I believe it is safe enough for me so I started taking metformin in October 2019. I bought it over the counter in Asia.