I have been diagnosed with this. It is quite debilitating and leaves me with very low energy levels, so I walk round very slowly like a geriatric.
The cause is that my body is unable to absorb B12 from food, so I have to have B12 injections. The deficiency leads to malformed red corpuscles...
well it is really a scandal that not all patients are informed of this sideeffect when offered Metformin
https://www.ncbi.nlm.nih.gov/pubmed/24959880
CONCLUSIONS:
The reduction of vitamin B12 may be induced by metformin in a dose dependent manner.