Vitamin B-12 helps maintain the health of your metabolism, red blood cells and nerves. When you have a vitamin B-12 deficiency, your doctor might prescribe vitamin B-12 shots, especially if you have an intestinal disorder that prevents the proper absorption of the vitamin. Your doctor might avoid giving you vitamin B-12 shots if you have allergies or certain medical conditions.
If you are allergic to cobalamin, cobalt or any ingredients in vitamin B-12 shots, inform your doctor before the vitamin is prescribed. Your doctor most likely will avoid prescribing vitamin B-12 shots to prevent a severe life-threatening allergic reaction. Seek immediate medical attention if you develop symptoms of an allergic reaction while on vitamin B-12 therapy, including breathing problems, wheezing, shortness of breath, hives, or swelling of the face, tongue and throat.
Your doctor might avoid prescribing vitamin B-12 shots if you have Leber's disease, an inherited eye disorder that causes damage to nerves in the eye, according to Drugs.com. If you take vitamin B-12 when you have Leber's disease, you might develop permanent optic nerve damage and blindness. Avoid taking OTC vitamin B-12 without consulting your doctor.
Your doctor also might avoid prescribing vitamin B-12 shots if you have a blood disorder called polycythemia vera, according to Drugs.com. Polycythemia vera occurs when there is an increased production of red blood cells in your body, which leads to increased blood thickness. Vitamin B-12 might worsen polycythemia vera because it stimulates production of red blood cells.
Vitamin B-12 injections are available as a liquid that usually is injected into deep muscles in your body. The shots usually are administered at the doctor's office. If B-12 deficiency is severe, you will receive B-12 shots once a day for six to seven days. When you begin to improve, the shots might be administered every two weeks or once a month. Attend all your doctor's appointments to receive vitamin B-12 shots to prevent developing complications such as permanent spinal cord damage.