High doses of vitamin C, popularly used as a home remedy against the common cold, destroy substantial amounts of vitamin B when ingested with food. Because this undesirable effect is unequivocal when measured in vitro, while the value of ascorbic acid in preventing the common cold is highly equivocal, daily ingestion of 0.5 gm or more of ascorbic acid without regular evaluation of vitamin B12 status is probably unwise.
(JAMA 230:241-242, 1974)
Herbert V, Jacob E. Destruction of Vitamin B12 by Ascorbic Acid. JAMA. 1974;230(2):241–242. doi:10.1001/jama.1974.03240020031018