My Iron levels tend to go a little low occasionally and Metformin can make it worse in some people.
I take a normal multivitamin pill primarily for Iron, Vitamin D and sublingual (under the tongue) vitamin B complex liquid (it is loaded with B12 in particular). If I stop taking the vitamin B for any length of time, I become a bit anaemic.
I've been experimenting with Chromium supplement and Cinnamon - so far, I think there may be a modest fall in after-meal BG (around 0.3) but this is controversial, to say the least, and not well-supported by much in the way of scientifically rigorous studies.
The vitamins were on medical advice, Chromium and Cinnamon are a personal decision.
Mark