Supplementing with B12 is probably not helpful in people who are able to get enough of the vitamin through their diet and have no difficulty with absorption.

Supplements that are available over the counter contain 500 to 2,000 μg of vitamin B12 per tablet. Long-term use is not recommended without monitoring of vitamin B12 levels.

For replenishing stores of vitamin B12, 1,000 μg should be given by intramuscular injection three times per week for two weeks. This can then be done weekly for four weeks, then once monthly. In cases where absorption is not an issue, the maintenance dose can also be given orally.^[1]

Vitamin B12 RDA

The recommended dietary allowance (RDA) is the amount of vitamin B12 that should be consumed daily to meet the nutritional needs of most people without health conditions. For infants (up to 12 months of age), an adequate intake value is given rather than an RDA, because there is limited data. It is based on the average amount of vitamin B12 obtained through breast milk.

Units: µg = micrograms. * Adequate intake (AI)
Reference: Institute of Medicine. Vitamin B₁₂ (chapter 9 in Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B₆, Folate, Vitamin B₁₂, Pantothenic Acid, Biotin, and Choline. The National Academies Press. 1998. DOI:10.17226/6015)

Vitamin B12 UL

Vitamin B12 is generally safe for adults, even at high doses. There is currently no established tolerable upper intake level (UL).^[2]