1. Clinical Overview
Vitamin D3 (Cholecalciferol) is a fat-soluble secosteroid hormone essential for calcium and phosphate homeostasis, bone mineralization, and numerous non-skeletal functions. In the Indian context, widespread deficiency due to factors like skin pigmentation, cultural clothing, dietary habits, and limited sun exposure makes it a critical therapeutic and prophylactic agent. It is available as a nutritional supplement and prescription medicine in various oral and parenteral formulations.
| Onset | Duration | Bioavailability |
|---|---|---|
| Serum 25-hydroxyvitamin D levels begin to rise within 24-48 hours of oral administration, but clinical effects on bone (e.g., reduced bone pain, improved mineralization) may take weeks to months. | Long-lasting. The half-life of stored vitamin D (as 25-hydroxyvitamin D) is approximately 15 days. Following a single large oral or intramuscular dose, effects can persist for several months. | Approximately 55-99% for oral formulations, depending on fat content of the meal (co-administration with a fatty meal increases absorption by ~30-50%). Intramuscular administration provides 100% bioavailability. |
2. Mechanism of Action
Vitamin D3 is a prohormone. It is converted to 25(OH)D in the liver and then to the active hormone 1,25(OH)2D in the kidneys. 1,25(OH)2D binds to the Vitamin D Receptor (VDR), a nuclear transcription factor. The VDR-Retinoid X Receptor (RXR) complex binds to Vitamin D Response Elements (VDREs) in target genes, modulating their expression.
3. Indications & Uses
- Prevention and treatment of Vitamin D deficiency
- Nutritional rickets and osteomalacia
- Osteoporosis (as an adjunct to specific therapy)
- Hypoparathyroidism (as part of management)
- Renal osteodystrophy in chronic kidney disease (stage 3-5)
4. Dosage & Administration
Adult Dosage: **Deficiency Treatment:** 60,000 IU once weekly for 8-12 weeks, followed by maintenance (e.g., 60,000 IU monthly or 1000-2000 IU daily). **Prevention:** 400-1000 IU daily or 60,000 IU once every 2-3 months. **Osteoporosis:** 800-2000 IU daily as adjunct.
Administration: Take oral capsules/tablets with a meal containing fat (e.g., milk, curd, nuts) to enhance absorption. Oral solution drops can be placed directly on the tongue or mixed with a small amount of milk/food. Intramuscular injections are administered deep IM, typically in the gluteal region.
5. Side Effects
Common side effects may include:
- Nausea
- Constipation
- Dry mouth
- Metallic taste
- Headache
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Thiazide Diuretics (e.g., Hydrochlorothiazide) | Increased risk of hypercalcemia due to reduced renal calcium excretion. | Major |
| Systemic Corticosteroids (e.g., Prednisolone) | May reduce calcium absorption and antagonize Vitamin D effects. | Moderate |
| Anticonvulsants (Phenobarbital, Phenytoin, Carbamazepine) | Induce hepatic CYP450 enzymes, increasing metabolism of Vitamin D to inactive compounds, leading to lower 25(OH)D levels. | Moderate |
| Orlistat, Cholestyramine | Reduce absorption of fat-soluble Vitamin D. | Moderate |
| Cardiac Glycosides (Digoxin) | Hypercalcemia may potentiate digoxin toxicity and risk of arrhythmias. | Major |
| Calcium Supplements | Additive risk of hypercalcemia and hypercalciuria. | Moderate |
| Ketoconazole, Antifungals | May inhibit the enzyme CYP27B1, reducing conversion to active form. | Minor |
7. Patient Counselling
- DO take with your largest meal of the day that contains fat (e.g., lunch with dal/curry containing oil).
- DO inform your doctor if you are taking any other vitamins, calcium pills, or diuretics (water pills).
- DO get regular sun exposure (10-30 minutes of midday sun on arms and legs, 2-3 times a week) if possible, but avoid sunburn.
- DON'T take more than the prescribed dose.
- DON'T start high-dose calcium supplements without consulting your doctor.
8. Toxicology & Storage
Overdose: Manifestations of hypervitaminosis D and hypercalcemia: Early - nausea, vomiting, constipation, anorexia, polydipsia, polyuria, weakness, headache. Late - confusion, psychosis, renal calculi, nephrocalcinosis, hypertension, cardiac arrhythmias, coma.
Storage: Store below 30°C. Protect from light and moisture. Keep oral drops, capsules, and tablets in their original container, tightly closed. Keep out of reach of children. Do not freeze.