A vitamin b12 injection (cyanocobalamin) is a synthetic form of vitamin b12. The primary reason for getting a b12 injection is to treat b12 deficiency. For many years b12 injections have also been given for patients suffering from fatigue and low energy. The most common dosage is a 1000 microgram (mcg) b12 injection once a week. There are no upper limit dosages to vitamin b12 and there are no reported side effects to b12 overdose.