1. Clinical Overview
Vitamin D3 (Cholecalciferol) 300,000 IU is a high-dose, single-use oral or intramuscular formulation of the secosteroid hormone used for rapid correction of severe vitamin D deficiency. It is a prohormone that undergoes hepatic and renal hydroxylation to form the active metabolite calcitriol (1,25-dihydroxycholecalciferol), which regulates calcium and phosphate homeostasis, bone mineralization, and modulates immune function.
| Onset | Duration | Bioavailability |
|---|---|---|
| Serum 25-hydroxyvitamin D (25(OH)D) levels begin to rise within 24-48 hours of oral administration. Peak serum concentrations of 25(OH)D are typically achieved 7-14 days post-dose. | The duration of effect is prolonged. A single 300,000 IU dose can maintain serum 25(OH)D levels above the sufficiency threshold (≥20 ng/mL) for approximately 8-12 weeks in deficient individuals, depending on baseline status, BMI, and individual metabolism. | Approximately 55-70% for oral solution (in oil-based vehicle) when taken with a fatty meal. Intramuscular bioavailability is considered near 100%. |
2. Mechanism of Action
Vitamin D3 is a prohormone. After conversion to its active form, calcitriol, it binds to the Vitamin D Receptor (VDR), a nuclear receptor. The VDR-ligand complex heterodimerizes with the Retinoid X Receptor (RXR) and binds to Vitamin D Response Elements (VDREs) in the promoter regions of target genes, modulating their transcription.
3. Indications & Uses
- Severe Vitamin D Deficiency (Serum 25(OH)D <10 ng/mL)
- Nutritional Rickets and Osteomalacia (for rapid correction)
- Vitamin D deficiency with symptomatic myalgia/bone pain
4. Dosage & Administration
Adult Dosage: A single oral dose of 300,000 IU, repeated only after assessment of serum 25(OH)D levels (typically after 8-12 weeks). For severe deficiency, some protocols use 300,000 IU monthly for 2-3 months. Intramuscular: 300,000 IU as a single deep IM injection.
Administration: ORAL: Administer the liquid/softgel capsule directly into the mouth or with a small amount of water. MUST be taken with or immediately after a meal containing fat (e.g., milk, curd, nuts) to maximize absorption. INTRAMUSCULAR: Administer as a deep IM injection, typically in the gluteal or deltoid muscle. Rotate sites if multiple doses are planned.
5. Side Effects
Common side effects may include:
- Nausea
- Constipation
- Dry mouth
- Metallic taste
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Thiazide Diuretics (e.g., Hydrochlorothiazide) | Increased risk of hypercalcemia due to reduced renal calcium excretion. | Major |
| Cardiac Glycosides (Digoxin) | Hypercalcemia may potentiate digitalis toxicity, leading to cardiac arrhythmias. | Major |
| Systemic Corticosteroids (e.g., Prednisolone) | May reduce the calcium-absorbing effects of vitamin D. | Moderate |
| Anticonvulsants (Phenytoin, Phenobarbital, Carbamazepine) | Increase hepatic metabolism of vitamin D to inactive compounds, reducing its efficacy. | Moderate |
| Orlistat, Cholestyramine | May reduce absorption of fat-soluble vitamin D. | Moderate |
| CYP3A4 Inducers (e.g., Rifampicin) | May increase metabolism of vitamin D, reducing serum levels. | Moderate |
7. Patient Counselling
- DO take the oral dose with a meal containing fat (e.g., milk, full-fat yogurt, nuts).
- DO inform your doctor if you are taking any other medicines, especially for heart, blood pressure, or seizures.
- DO get your blood test (Vitamin D, Calcium) done as advised by your doctor, typically 8-12 weeks after this dose.
- DO follow up with a daily maintenance dose of Vitamin D as prescribed.
- DONT take additional Vitamin D supplements or high-dose calcium supplements without consulting your doctor.
- DONT take this high dose repeatedly on your own; it is not a monthly supplement.
8. Toxicology & Storage
Overdose: Manifestations of vitamin D toxicity are those of hypercalcemia: Nausea, vomiting, constipation, anorexia, weakness, weight loss, confusion, polydipsia, polyuria, nephrocalcinosis, cardiac arrhythmias, and in severe cases, coma and death.
Storage: Store below 25°C. Protect from light and moisture. Keep the oral solution/softgel in the original container. Keep out of reach of children. Do not freeze.