Vitamin B12 is generally well tolerated by most people if taken in recommended amounts. Side effects may include mild diarrhea, itching, blood clots, urine discoloration, and allergic reactions.
People with Leber's disease (a genetic eye disease) or those who are allergic or sensitive to cobalt or cobalamin should consult their health care provider before taking vitamin B12 supplements.
People with anemia need to talk to their health care provider about what treatment is right for them before starting any type of vitamin supplementation. Taking vitamin B12 supplements before a proper diagnosis from the doctor may make it harder for your doctor to diagnose the type of anemia you have.
Vitamin B12 is safe for pregnant or breast-feeding women when taken by mouth in recommended amounts.
Vitamin B12 from food sources can interact with certain medications, including proton pump inhibitors (e.g., omeprazole, pantoprazole), H2 blockers (e.g., famotidine, ranitidine), and metformin. These can affect the absorption of dietary B12 into the body, though this does not appear to be a problem with supplements. Heavy drinking for more than 2 weeks can also decrease vitamin B12 absorption.
Avoid taking the combination of vitamin B12 and chloramphenicol (an antibiotic) since chloramphenicol can destroy the newly produced blood cells that vitamin B12 helped to make.
Vitamin C and potassium supplements can decrease the effects of vitamin B12. It is not known whether this interaction is clinically significant or not. You can avoid this by separating the vitamins and taking them at least 2 hours apart. Consult your health care provider for more information on drug interactions.
Before taking any new medications, including natural health products, speak to your physician, pharmacist, or other health care provider. Tell your health care provider about any natural health products you may be taking.