One of the problems with Metformin’s actions is that it causes vitamin B12 deficiency.
This side effect has been known since 2006. By way of the same mechanism that blocks sugar absorption by the intestines, this essential vitamin is also blocked. The extent of the deficiency is dose- and time-dependent: the higher the amount you take and the longer you take Metformin, the greater and more critical the deficiency.
The Diabetes Drug Metformin Linked to Vitamin B12 Deficiency