Feeling Tired All the Time? It Might Be a Vitamin B12 Deficiency Fusion HealthCare Vitamin B 12 intramuscular injections are also used therapeutically to help restore energy and combat various symptoms related to such deficiencies. Vitamin B 12 is essential to the development of healthy blood cells, nerve Vitamin B12 Injection Side Effects and Safety Can Vitamin B12 Deficiency Indicate Cancer? Know Warning Signs Vitamin b12 deficiency can cause several symptoms including, extreme tiredness, lack of energy, mental fog, and changes in mood and balance. Find out more about b12 deficiency and our private vitamin b12