Metformin is the first line of defence for Diabetes. It is the old workhorse and perhaps it is the only diabetic medicine which has least side effects.
While going through this beautiful article on " Drug induced nutrient Depletion" which is highly relevant to all diabetes, metformin is listed as depleting B12 levels and also Coq10.
If any one is on metformin, he needs to check his Vitamin B12 level and Coq10 level and if found deficient he should take supplement of methylcobalamine and folic acid after consulting his doctor.Nowadays many pharma companies are making combo drugs of metformin and methyl cobalamine e.g Bigomet from Aristo, Sugamet from Zydus , Formin plus from Alkem to name a few.
Does not matter whether he is a Veg or non Veg, B12 deficiency could be caused by long term use of Metformin.
Lack of B12 and Coq could lead to peripheral neuropathy, fatigue leg, muscle pain.
Please read on this article which has listed many drugs such as BP lowering , Cholesterol lowering drugs could depletes nutrients.