Side effects of B12 cyanocobalamin injection include itching and mild diarrhea, according to WebMD. The patient may also feel as if their entire body is swollen, reports MedlinePlus.
Serious side effects are uncommon, but they include extreme fatigue, extreme thirst, and pain and swelling in one leg, according to MedlinePlus. Patients should consult with their doctors or pharmacists when they experience these side effects and others, such as frequent urination, shortness of breath and swelling in the legs, arms and feet.
B12 cyanocobalamin injections may cause low potassium levels, or hypokalemia, explains WebMD. This causes side effects such as muscle cramps, fast heart rate and extreme weakness. Individuals who suffer from the bllod disorder polycythemia vera may experience the symptoms of this disorder when they take the B12 cyanocobalamin injection.
Other side effects of B12 cyanocobalamin injection include headaches, dizziness, confusion and reddening of the skin, especially on the face. Allergic reactions are uncommon, notes WebMD. However, individuals who are allergic to cyanocobalamin may experience certain reactions, such as trouble breathing, rashes, hives and swelling of the face, tongue or throat.
Side effects that affect the nervous system include anxiety, abnormal gait, incoordination and nervousness, reports Drugs.com. Musculoskeletal side effects include back pain and myalgia, while immunological side effects include infection.