There is abundant evidence that metformin, a medication commonly used for type 2 diabetes, impairs the absorption of Vitamin B12 (even by broad pathological reference standards, not to mention the more sensitive functional reference ranges). Few patients seem to be told about this even though it has been well known for a long time:

1. Use of Metformin Is a Cause of Vitamin B₁₂ Deficiency
2. Risk Factors of Vitamin B112 Deficiency in Patients Receiving Metformin
3. Metformin-related vitamin B12 deficiency
4. Metformin increases total serum homocysteine levels in non-diabetic male patients
5. Malabsorption of vitamin B12 during biguanide therapy
6. Association of metformin, elevated homocysteine and methylmalonic acid

For vitamin B12 I recommend the sublingual methylcobalamin form.

(Metformin is a type of biguanide. Homocysteine and methylmalonic acid levels go up with B12 deficiency.)

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