The quickest approach for your body to take Vitamin B12 is through B12 injections, which also have a high absorption rate. Your body will start to produce new red blood cells in 48 to 72 hours. To get the greatest benefit from two to three injections over several weeks for minor deficits. A doctor may recommend vitamin B12 injections to address a vitamin B12 shortage, particularly if the patient's body has trouble absorbing the vitamin. The effects of a vitamin B12 shortage might range from fatigue to long-term neurological damage. If a person does not consume animal products or if their body does not naturally absorb vitamin B12 properly, they may be deficient in it. B12 injections are frequently recommended for those who are vitamin B12 deficient.