I took a general mutivit and mineral supplement at the start when my eating was extremely reduced as a result of metformin nausea, but a later blood test (after about 9 months) revealed that all my levels were in rude good health, with a tad too much iron, so I stopped taking them. My diet had stabilised by that point and I was eating good, natural food. I also took a vit B12 while I was on metformin, because metformin can affect uptake of this vitamin. It is apparently best to take methyl (or methylated) B12 as it’s the most easily absorbed and utilised.
I do take vit D (because it’s a common deficiency in northern lands) and a slow-release magnesium for leg cramps (a common side effect of a low carb diet).