The Recommended Daily Allowance (RDA) of vitamin B12 is 2.4 μg for adults without known health conditions. During pregnancy, the RDA increases to 2.6 μg.^[1] For children 8 years and under, the RDA is 1.2 μg.^[1] 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 500–2000 μg of vitamin B12 per tablet. Long-term use is not recommended without monitoring of vitamin B12 levels.

For replenishing stores of vitamin B12, 1000 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.^[2]