Adverse Reactions to Vitamin B12 Shots

Vitamin B-12 is a water soluble vitamin that is naturally found in animal foods such as meat, poultry, fish, milk and shellfish. Vitamin B-12 is important for the production of red blood cells and for the maintenance of healthy nerves. Lack of vitamin B-12 can lead to neurological damage and pernicious anemia. Patients who experience vitamin B-12 deficiency are treated using B-12 shots. Some patients may experience adverse reactions after B-12 shots.

Heart Failure and Blood Clots

Vitamin B-12 shots are administered directly into the bloodstream through intramuscular injections, according to Vitamin B-12 may prevent the heart from beating properly leading too symptoms of heart failure such as sudden weight gain, problems breathing due to accumulation of fluids in the lungs, edema, chest pain, fatigue, cough, palpitations and shortness of breath. Vitamin B-12 shots may also cause thrombosis or blood clots in the extremities. Blood clots are dangerous because they can dislodge and travel to the heart and brain causing heart attack and stroke.

Polycythemia Vera

Use of vitamin B-12 can also unmask a blood disorder known as polycythemia vera, according to MayoClinic. Polycythemia vera is characterized by an abnormal increase in the production of red blood cells and increase in the volume of blood. Symptoms of polycythemia include fatigue, dizziness, reddish color on the face, shortness of breath and problems breathing especially when lying down. Patients with a history of polycythemia vera should avoid B-12 shots.


Vitamin B-12 may cause blindness in patients with Leber's disease, according to MayoClinic. Leber's disease is a genetic disease characterized by atrophy of the optic nerve. Vitamin B-12 use should be avoided in patients with this eye disorder.


Patients allergic to cyanocobalamin, cobalamin, cobalt and other ingredients in vitamin B-12 shots may develop severe allergic reactions characterized by swelling of the face and tongue, hives, rashes, itching, chest tightness, problems breathing and chest pain.

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