Vitamin D and Metformin

Metformin is the most-prescribed diabetes drug worldwide. It's often combined with vitamin D supplementation — both are cheap, safe, and addressed together in prediabetes, T2D, and PCOS care. The key interaction to know: metformin depletes vitamin B12.

B12 depletion — the well-established interaction

Chronic metformin use (~4+ years) reduces serum B12 by ~10-30% in long-term users. Prevalence of B12 deficiency in metformin users: 5-10% overall, higher in older adults. American Diabetes Association recommends periodic B12 screening in long-term metformin users. This is not a vitamin D issue but is worth knowing. See our B12 page.

Vitamin D and metformin combined

No direct pharmacological interaction. Vitamin D + metformin combinations are common in:

• PCOS — vitamin D deficiency is common; metformin is standard care.
• Prediabetes — D2d trial tested vitamin D for T2D prevention; metformin is DPP evidence-based.
• T2D — vitamin D correction alongside standard diabetes care.
• Gestational diabetes.

Practical care

1. Long-term metformin users: check B12 every 2 years; more often if elderly, neuropathy, or anaemia.
2. Vitamin D 1,000-2,000 IU/day standard dosing; correct any deficiency.
3. Some patients benefit from a B12 supplement (500-1,000 µg oral cyanocobalamin) alongside metformin.
4. The evidence-based diabetes prevention/treatment is metformin (or GLP-1 agonists, SGLT2 inhibitors) plus lifestyle change — vitamin D is adjunctive.