Cyanocobalamin prescription

Prescription Writing Rules

In Latin and pharmacology classes, you are required to be able to prescribe vitamin $B_{12}$ in various dosage forms. Note that the name of the active substance is written with a capital letter in the genitive case — Cyanocobalamini. The drug is prescribed until complete correction of the anemia.

Parenteral form (solution in ampoules): Rp.: Sol. Cyanocobalamini 0.05% – 1 ml D.t.d. N. 10 in ampullis S. 1 ml intramuscularly every other day until correction of the anemia.

Oral form (coated tablets): Rp.: Tab. Cyanocobalamini obductae 1000 mcg D.t.d. N. 30 S. 1 tablet once a day orally until correction of the anemia.

Pharmacodynamics and mechanism of action

For cyanocobalamin to be absorbed and exert its effect, it must bind in the stomach to a special protein — intrinsic factor (Castle's factor). After successful absorption, the vitamin is transported in the systemic circulation by specific proteins — transcobalamins (types I and II).

In body tissues, the drug is converted into its active coenzyme forms: methylcobalamin and cobamamide. They play a key role in several processes:

• Synthesis of tetrahydrofolic acid.
• Formation of purine and pyrimidine bases, which are critically important for DNA and RNA synthesis.
• Synthesis of myelin, which forms the protective sheaths of nerve fibers.

Clinical presentation of deficiency and dosing regimens

Vitamin $B_{12}$ deficiency manifests with a classical clinical triad:

1. Hematological syndrome: development of megaloblastic anemia.
2. Gastroenterological syndrome: appearance of glossitis and atrophic gastritis.
3. Neurological syndrome: polyneuritis and severe spinal cord damage — funicular myelosis.

The standard treatment regimen involves parenteral administration of 100–1000 mcg of the drug per day every other day. However, if the patient already has neurological disorders, the dosage is changed: 400–1000 mcg per day is prescribed daily during the first week of therapy. An alternative is oral administration of 1000 mcg daily.

Combination therapy and folic acid

Treatment is often supplemented with folic acid preparations (vitamin $B_9$). In the body, it is converted into tetrahydrofolic acid, which serves as a methyl group donor, participates in amino acid synthesis, and stimulates the transition of pathological megaloblasts to normal normoblasts.

Folic acid prescription (in combination with $B_{12}$): Rp.: Acidi folici 0.005 D.t.d. N. 30 in tabulettis S. 1 tablet once a day orally for 20–30 days.

Important rule: in pernicious ($B_{12}$-deficient) anemia, it is strictly forbidden to prescribe folic acid in isolation. It can eliminate hematological manifestations (anemia), but at the same time masks and significantly worsens irreversible neurological disorders.

Contraindications to administration also differ: for cyanocobalamin, these are conditions accompanied by hemoconcentration (erythremia, erythrocytosis, thromboembolism). For folic acid, a strict restriction is the patient's age under 3 years.

Class and exam questions

What else to study on this topic

• The role of Castle's intrinsic factor and transcobalamins in pharmacokinetics
• The mechanism of development of funicular myelosis in cobalamin deficiency
• Differences between active forms: methylcobalamin and cobamamide
• Biochemical relationship between vitamins B12 and B9 in DNA synthesis
• Differential diagnosis of megaloblastic anemias

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.