The correct dosage for vitamin B-12 injections depends on the patient's age and the reason for the injection, states Mayo Clinic. For adults with vitamin B-12 deficiencies, doctors often prescribe 1,000 microgram intramuscular injections of cobalamin daily for 10 days, and then reduce the dosage to weekly for four weeks.
After four weeks, patients with vitamin B-12 deficiencies can expect to take B-12 injections permanently, explains Mayo Clinic.
Doctors treating adult depression administer 1 milligram injections of cyanocobalamin weekly for four weeks. For mental performance, doctors prescribe one 1,000 microgram vitamin B-12 injection daily for five days, followed by one monthly injection of 1,000 micrograms for five months. Doctors also administer 1,000 microgram injections weekly for four weeks, or once a month for six months, notes Mayo Clinic.