Pain/redness at the injection site, mild diarrhea, itching, or a feeling of swelling all over the body may occur. If any of these effects persist or worsen, tell your doctor or pharmacist promptly.
Remember that your doctor has prescribed this medication because he or she has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects.
This medication may cause low potassium levels in the blood (hypokalemia). Tell your doctor right away if any of these unlikely but serious side effects occur: muscle cramps, weakness, irregular heartbeat.
People who have a rare blood disorder (polycythemia vera) may rarely have symptoms related to this disorder while taking cyanocobalamin. Seek immediate medical attention if any of these rare but very serious symptoms occur: chest pain (especially with shortness of breath), weakness on one side of the body, sudden vision changes, slurred speech.
Get medical help right away if you have any very serious side effects, including: symptoms of heart failure (such as shortness of breath, swelling ankles/feet, unusual tiredness, unusual/sudden weight gain).
A very serious allergic reaction to this drug is rare. However, seek immediate medical attention if you notice any symptoms of a serious allergic reaction, including: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing.
This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist.
In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch.
In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.