Does metformin really deplete B12? When should I start supplementing?

I've been on metformin 1000mg ER daily for 8 weeks (type 2 diabetes, A1C now 6.8%). My doctor mentioned B12 depletion risk but didn't order labs. I'm also on bupropion XL 300mg for depression. Should I start B12 supplementation now, or wait for labs? What form—oral, sublingual, or injection? I'm vegetarian, so baseline intake is already low.

⚕ Pinned FDA / NIH / PubMed Clinical Context • Nourhan Mansour — Literature & Label Auditor

Metformin reduces B12 absorption in 10–30% of long-term users via calcium-dependent ileal uptake inhibition (PubMed 23539344). ADA recommends baseline B12 + annual monitoring; supplementation if <300 pg/mL or symptoms (paresthesia, fatigue).

Connected Clinical Guides: Condition: Type 2 Diabetes Condition: Diabetes Type 2 Condition: Prediabetes Rx: Metformin Rx: Metformin Hcl Rx: Tirzepatide Supplement: Vitamin B12 Supplement: Magnesium Glycinate Supplement: Berberine

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Verified Peer Replies (2)

Nourhan Mansour — Literature & Label Auditor

Get baseline serum B12 + methylmalonic acid (MMA) now—vegetarian status + metformin = dual risk. If B12 <300 pg/mL or MMA elevated, start cyanocobalamin 1000 mcg weekly IM or sublingual 2000 mcg daily. Oral absorption is unreliable on metformin; sublingual bypasses ileal uptake.

Female • Age 45-59 • 1000mg ER • Week 52

Waited 6 months, then labs showed B12 at 285 pg/mL. Started sublingual 2000 mcg daily. Fatigue and brain fog lifted in 3 weeks. Recheck labs every 6 months now. Bupropion didn't interact—B12 depletion was purely metformin-driven.