Usage
Vitamin B12 is prescribed for the prevention and treatment of vitamin B12 deficiency. This deficiency can lead to megaloblastic anemia, neurological problems (numbness, tingling, nerve damage), fatigue, weakness, and other health issues. It’s pharmacologically classified as a water-soluble vitamin, specifically one of the B vitamins.
Vitamin B12 is crucial for DNA synthesis, red blood cell formation, and the maintenance of a healthy nervous system. It acts as a coenzyme for two essential enzymes: methionine synthase (involved in DNA and protein synthesis) and methylmalonyl-CoA mutase (important for fatty acid metabolism).
Alternate Names
- Cobalamin
- Cyanocobalamin (a common supplemental form)
- Hydroxocobalamin (another supplemental form)
- Methylcobalamin (a supplemental form)
- Adenosylcobalamin (a supplemental form)
How It Works
Pharmacodynamics: Vitamin B12 plays vital roles in cell division and growth, red blood cell maturation, nerve function, and the conversion of food into energy. Deficiency disrupts these processes, leading to anemia, neurological damage, and other problems.
Pharmacokinetics:
- Absorption: Vitamin B12 in food is bound to proteins. In the stomach, hydrochloric acid and pepsin release B12 from these proteins. It then binds to intrinsic factor, a protein produced in the stomach, and is absorbed in the ileum (the last part of the small intestine).
- Distribution: B12 is transported in the blood bound to transcobalamin II and is stored primarily in the liver.
- Metabolism: B12 is converted into its active coenzyme forms, adenosylcobalamin and methylcobalamin.
- Elimination: Excess B12 is excreted in the urine.
Mode of Action: Vitamin B12 functions as a coenzyme for methionine synthase and methylmalonyl-CoA mutase. Methionine synthase is essential for DNA synthesis and the production of the amino acid methionine. Methylmalonyl-CoA mutase is important for the metabolism of certain fatty acids and amino acids.
Receptor Binding, Enzyme Inhibition, or Neurotransmitter Modulation: Vitamin B12’s primary action is as a coenzyme rather than through receptor binding, enzyme inhibition, or direct neurotransmitter modulation. However, its role in the synthesis of methionine and other essential molecules indirectly influences various metabolic pathways and nervous system function.
Elimination Pathways: Primarily renal excretion.
Dosage
Standard Dosage
Adults:
- Recommended Dietary Allowance (RDA): 2.4 mcg/day
- Deficiency treatment: Oral supplements of 1000-2000 mcg daily or injections of 1000 mcg daily for 1 week, then weekly for 1 month, followed by monthly injections. Doses may vary depending on the severity of deficiency and individual response.
Children:
- RDA: Varies depending on age, starting from 0.4 mcg/day for infants under 6 months.
- Deficiency treatment: Dosages are determined based on the child’s age and weight.
Special Cases:
- Elderly Patients: Due to decreased absorption with age, higher doses or fortified foods are recommended.
- Patients with Renal Impairment: Dose adjustment may be necessary based on creatinine clearance.
- Patients with Hepatic Dysfunction: Dose adjustments typically aren’t needed.
- Patients with Comorbid Conditions: Consider potential drug interactions with medications for other conditions.
Clinical Use Cases
Dosage for these specific settings varies greatly and should be managed by a healthcare professional according to individual needs.
- Intubation: Not specifically indicated.
- Surgical Procedures: Not typically part of standard surgical protocols.
- Mechanical Ventilation: Not specifically required.
- Intensive Care Unit (ICU) Use: May be administered for identified deficiencies.
- Emergency Situations: Not routinely used.
Dosage Adjustments
Dose adjustments may be needed for specific medical conditions, including malabsorption syndromes, pernicious anemia, and following gastric surgery. Monitoring vitamin B12 levels is crucial to ensure adequate treatment.
Side Effects
Common Side Effects
Generally well-tolerated. High doses may cause mild gastrointestinal upset (diarrhea, nausea).
Rare but Serious Side Effects
Allergic reactions (rash, itching, swelling), pulmonary edema, heart failure (rarely with injections), hypokalemia.
Long-Term Effects
Polycythemia vera (rare)
Adverse Drug Reactions (ADR)
Anaphylaxis (rare).
Contraindications
Hypersensitivity to cobalamin or cobalt, Leber’s hereditary optic neuropathy.
Drug Interactions
- Metformin: May reduce B12 absorption.
- Proton pump inhibitors (PPIs): May decrease B12 absorption.
- H2 receptor blockers: May reduce B12 absorption.
- Aminosalicylic acid: Decreased B12 absorption.
- Colchicine: May impair B12 absorption.
- Chloramphenicol: May interfere with B12 utilization.
- High doses of vitamin C: May decrease B12 availability.
Pregnancy and Breastfeeding
Vitamin B12 is considered safe during pregnancy and breastfeeding at recommended doses. Higher doses are generally safe but should be used under medical supervision. Adequate B12 intake is important for fetal development and infant health.
Drug Profile Summary
- Mechanism of Action: Coenzyme for methionine synthase and methylmalonyl-CoA mutase.
- Side Effects: Generally well-tolerated; rare allergic reactions, hypokalemia, pulmonary edema, heart failure.
- Contraindications: Hypersensitivity to cobalamin or cobalt; Leber’s hereditary optic neuropathy.
- Drug Interactions: Metformin, PPIs, H2 blockers, aminosalicylic acid, colchicine, chloramphenicol, high-dose vitamin C.
- Pregnancy & Breastfeeding: Safe at recommended doses.
- Dosage: Adults: 2.4 mcg/day; deficiency: 1000-2000 mcg orally or 1000 mcg injection (doses and regimens vary). Children: Dose according to age.
- Monitoring Parameters: Serum B12 levels, complete blood count (CBC), potassium levels (with injections).
Popular Combinations
Often included in B-complex vitamins and multivitamins. May be combined with folic acid, especially for treating deficiencies.
Precautions
- General Precautions: Monitor for allergic reactions. Check potassium levels during initial treatment with high-dose injections.
- Specific Populations: Consider risk factors for deficiency in vegans/vegetarians, elderly, those with gastrointestinal disorders.
FAQs (Frequently Asked Questions)
Q1: What is the recommended dosage for Vitamin B12?
A: The RDA for adults is 2.4 mcg/day. Therapeutic dosages for deficiency vary based on severity and route of administration, ranging from 1000-2000 mcg/day orally or 1000 mcg injections.
Q2: How is vitamin B12 deficiency diagnosed?
A: Through blood tests measuring serum B12 levels, complete blood count (CBC), methylmalonic acid (MMA) levels, and homocysteine levels.
Q3: What are the common symptoms of B12 deficiency?
A: Fatigue, weakness, numbness and tingling in the extremities, difficulty walking, anemia, cognitive changes, and mood disturbances.
Q4: What are the main sources of vitamin B12?
A: Animal products (meat, fish, poultry, eggs, dairy) and fortified foods.
Q5: What are the different routes of administration for B12 supplements?
A: Oral, sublingual, intramuscular injection, and nasal spray.
Q6: What are the risks of long-term B12 deficiency?
A: Irreversible nerve damage, dementia, and other neurological problems.
Q7: Are there any drug interactions I should be aware of with B12?
A: Yes, certain medications like metformin, PPIs, and some antibiotics can interfere with B12 absorption.
Q8: Is B12 safe during pregnancy and breastfeeding?
A: Yes, it is generally considered safe at recommended doses and is important for fetal development and infant health.
Q9: What is the difference between cyanocobalamin and hydroxocobalamin?
A: Both are forms of B12 used in supplements. Hydroxocobalamin is retained in the body longer and may require less frequent injections.
Q10: Can high doses of B12 be harmful?
A: While B12 is generally considered safe even in large doses, very rare side effects like allergic reactions and hypokalemia have been reported. It is always best to follow medical advice regarding dosage.
