Actually, a single larger dose of vitamin B12 would be fine although absorption decreases as the dose increases. Monitoring serum B12, holotranscobalamin, MCV, and homocysteine will help determine when someone is replete.
A monthly intramuscular dose of 1000 ug (1 mg) vitamin B12 is often used to treat pernicious anemia. The intramuscular route bypasses intestinal absorption which is impaired in this condition.
Oral doses greater than 50 ug will induce some passive absorption without the need for the intrinsic factor. Recommended bioavailable forms include methylcobalamin, hydroxocobalamin, and adenosylcobalamin.
Dose Absorption
B12 less than 0.5 ug 50% (0.25 ug)
1 ug 20% (0.2 ug)
500 ug 1-1.2% (5-6 ug)
1000 ug 0.01- 0.012% (10-12 ug)
The RDA for vitamin B12 is 2.4 ug/day for non-pregnant, non-lactating adults. No Tolerable Upper Limit (UL) has been set for B12 since no adverse effects have been reported at high doses.
Research
Allen, Lindsay H et al. “Biomarkers of Nutrition for Development (BOND): Vitamin B-12 Review.” The Journal of nutrition vol. 148,suppl_4 (2018): 1995S-2027S. doi:10.1093/jn/nxy201
Smith, A David et al. “Vitamin B12.” Advances in food and nutrition research vol. 83 (2018): 215-279. doi:10.1016/bs.afnr.2017.11.005
Langan, Robert C, and Andrew J Goodbred. “Vitamin B12 Deficiency: Recognition and Management.” American family physician vol. 96,6 (2017): 384-389.
NIH ODS. Vitamin B12 Fact Sheet for Consumers. Retrieved October 22, 2020 from
Paul, Cristiana, and David M Brady. “Comparative Bioavailability and Utilization of Particular Forms of B12 Supplements With Potential to Mitigate B12-related Genetic Polymorphisms.” Integrative medicine (Encinitas, Calif.) vol. 16,1 (2017): 42-49.