The recommended dose for a vitamin B-12 shot is 0.4 micrograms for infants up to 6 months and 0.5 micrograms for infants aged 7 to 12 months, explains the National Institute of Health. The dose for children aged 1 to 3 years is 0.9 milligrams.
Children aged 4 to 8 years should take 1.2 micrograms of vitamin B-12, while those between the ages of 9 to 13 years should receive 1.8 micrograms, explains the National Institute of Health. Teens between 14 to 18 years should take 2.4 micrograms, adults 2.4 micrograms, pregnant women 2.6 micrograms and breast-feeding mothers 2.8 micrograms. Adults over 50 years should meet recommended dietary amounts of vitamin B-12 by taking supplements or eating foods reinforced with vitamin B-12, states Mayo Clinic.