High-dose vitamin C may reduce B12 absorption
Very high doses of vitamin C (>1000mg) taken simultaneously may reduce B12 absorption.
Separate high-dose vitamin C from B12 supplements.
Separate by 2+ hours if taking high-dose vitamin C
High-dose vitamin C may reduce B12 absorption. Separate high-dose vitamin C from B12 supplements.
Separate by 2+ hours if taking high-dose vitamin C
This interaction is rated “Caution” by Formulate. Very high doses of vitamin C (>1000mg) taken simultaneously may reduce B12 absorption.
Studies from our registry that mention vitamin b12 or vitamin c. Each links to the primary source and the other Formulate pages citing it.
Mentions vitamin b12 · 2014
The prevalence of cobalamin deficiency among vegetarians assessed by serum vitamin B12
Pawlak et al. · European Journal of Clinical Nutrition
Systematic review finding B12 deficiency prevalence of 62% in pregnant vegetarian women, 86% in elderly vegetarians, 41% in adolescents.
Mentions vitamin b12 · 2013
Vitamin B12 deficiency
Stabler SP · New England Journal of Medicine
Comprehensive clinical review of B12 deficiency diagnosis and treatment, noting that serum B12 alone is unreliable and that methylmalonic acid (MMA) and homocysteine are more sensitive markers of functional deficiency.
Mentions vitamin c · 2010
Iron bioavailability and dietary reference values
Hurrell R, Egli I · American Journal of Clinical Nutrition
Reviews enhancers (vitamin C, meat) and inhibitors (phytate, polyphenols, calcium) of non-heme iron absorption, providing the mechanistic basis for pairing iron with vitamin C and separating it from tea/coffee.
Mentions vitamin b12 · 2005
Oral cyanocobalamin supplementation in older people with vitamin B12 deficiency: a dose-finding trial
Eussen SJ, de Groot LC, Clarke R, et al. · Archives of Internal Medicine
Dose-finding RCT in older adults found 647–1,032 µg/day oral B12 was needed to normalize mild B12 deficiency markers — orders of magnitude above the RDA, supporting high-dose oral as an alternative to injections.
Mentions vitamin c · 2000
Prediction of dietary iron absorption: an algorithm for calculating absorption and bioavailability
Hallberg and Hulthén · American Journal of Clinical Nutrition
Foundational model for non-heme iron absorption factoring vitamin C, phytates, tannins, and calcium interactions.
Guide
Vitamin B12 Guide 2026: Which Form & Dose You Need
Find out if you’re actually B12 deficient — and why standard tests miss it. Covers methylcobalamin vs cyanocobalamin, dosing, and the MMA test.
Roundup
Best Vitamin C Supplements 2026: Form, Dose, and What Actually Works
Ascorbic acid, buffered, liposomal, or whole-food? Dose by goal (daily health, acute illness, iron absorption, collagen). What the Linus Pauling megadose narrative got wrong.
Review
Methylcobalamin vs Cyanocobalamin: Which B12 Form Actually Works
Most B12 supplements use cyanocobalamin — but methylcobalamin is the bioactive form. Here's when the difference matters and when it doesn't.
Other interactions to know
Long-term antacid use depletes B12
Long-term PPI use reduces B12 absorption
Vitamin C dramatically enhances iron absorption
Vitamin C regenerates quercetin
B12 and folate work together in methylation
Vitamin C is required for collagen synthesis
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Medical disclaimer. This page is for educational purposes only and does not replace advice from a qualified healthcare provider.
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