B12 Supplements (MAJOR EFFECT) Oral B12 Supplements: Typical dose: 1000 mcg (1 mg) Absorption: ~1-2% passively absorbed (10-20 mcg from 1000 mcg dose) Effect on serum B12: Can increase by 100-300 pg/mL within 24 hours Peak effect: 4-8 hours after oral supplement Intramuscular B12 Injections: Dose: Usually 1000 mcg Effect: Massive increase in serum B12 Can reach 1000-5000 pg/mL within hours Sustained elevation for weeks Recommendation: Stop oral supplements 3-7 days before testing (ideally 7 days) Wait 1-2 weeks after injection before testing 2
The semaglutide 5mg/2ml dosage chart provides a complete reference for this concentration, and the semaglutide dosage calculator can verify your math instantly
doi: 10.3389/fphar.2025.1602976 Received 30 March 2025 Revised 13 November 2025 Accepted 14 November 2025 Published 19 December 2025 Volume 16 - 2025 Edited by Bernd Rosenkranz, Fundisa African Academy of Medicines Development, South Africa Reviewed by Kacper Nijakowski, Poznan University of Medical Sciences, Poland Miriam Njoki Karinja, Science for Africa Foundation, Kenya Updates Copyright 2025 Mazur, Ndokaj, Salerno, Vallone, Ardan, Bietolini, Carrouel, Wilk, Sarig, Ottolenghi and Bourgeois
This is since shots allow the body to absorb vitamin B12 without going through the digestive system
