B-12 Side Effects
Generic name: cyanocobalamin
Medically reviewed by Drugs.com. Last updated on Sep 26, 2022.
Note: This document contains side effect information about cyanocobalamin. Some dosage forms listed on this page may not apply to the brand name B-12.
Applies to cyanocobalamin: intramuscular solution. Other dosage forms:
Serious side effects of B-12
Along with its needed effects, cyanocobalamin (the active ingredient contained in B-12) may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.
Check with your doctor or nurse immediately if any of the following side effects occur while taking cyanocobalamin:
Incidence not known
- Abdominal or stomach pain
- bleeding from the gums or nose
- blue lips and fingernails
- chest pain
- coughing that sometimes produces a pink frothy sputum
- decreased urine output
- difficult, fast, noisy breathing, sometimes with wheezing
- difficulty with swallowing
- dilated neck veins
- extreme fatigue
- eye pain
- fast heartbeat
- hives, itching, or skin rash
- increased sweating
- irregular breathing
- irregular heartbeat
- pale skin
- puffiness or swelling of the eyelids or around the eyes, face, lips, or tongue
- ringing in the ears
- swelling of the face, fingers, feet, or lower legs
- tightness in the chest
- unusual tiredness or weakness
- weight gain
Other side effects of B-12
Some side effects of cyanocobalamin may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects.
Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:
Incidence not known
- skin rash with a general disease
For Healthcare Professionals
Applies to cyanocobalamin: compounding powder, injectable solution, intramuscular solution, nasal gel, nasal spray, oral tablet, oral tablet extended release, sublingual liquid, sublingual lozenge, sublingual tablet.
Patients sensitized to the injectable product are often able to tolerate the oral route without problems.[Ref]
Frequency not reported: Severe dizziness, drowsiness, muscular paralysis, loss of consciousness[Ref]
Frequency not reported: Itching, transitory exanthema, acneiform eruptions, bullous eruptions[Ref]
Frequency not reported: Vision problems[Ref]
Frequency not reported: Feeling of swelling of entire body[Ref]
More about B-12 (cyanocobalamin)
Related treatment guides
1. Cerner Multum, Inc. "Australian Product Information." O 0
2. "Product Information. Cyanocobalamin (cyanocobalamin)." West Ward Pharmaceutical Corporation (2017):
3. Cerner Multum, Inc. "UK Summary of Product Characteristics." O 0
Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.
Some side effects may not be reported. You may report them to the FDA.