Get emergency medical help if you have signs of an allergic reaction: hives; difficulty breathing; swelling of your face, lips, tongue, or throat.
Call your doctor at once if you have:
easy bruising or bleeding (nosebleeds, bleeding gums);
extreme tiredness;
a light-headed feeling, like you might pass out;
pale skin, blue lips or fingernails;
eye pain, vision problems;
headache, ringing in your ears;
chest pain or tightness, fasts heartbeats;
cough, wheezing, trouble breathing;
trouble swallowing;
little or no urination;
heart problems--swelling, rapid weight gain, feeling short of breath;
low potassium level--leg cramps, constipation, irregular heartbeats, fluttering in your chest, increased thirst or urination, numbness or tingling, muscle weakness or limp feeling; or
signs of a blood clot in an arm or leg--pain, numbness, coldness, warmth, or pale appearance.
Common side effects may include:
diarrhea; or
rash.
This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.
 Cyanocobalamin side effects (more detail)
