1. Clinical Overview
Vitamin D3 (Cholecalciferol) 800 IU is a fat-soluble vitamin supplement used for the prevention and treatment of vitamin D deficiency. In the Indian context, widespread deficiency due to factors like skin pigmentation, dietary habits, and limited sun exposure makes it a critical therapeutic agent. It is a prohormone that undergoes hepatic and renal hydroxylation to form the active metabolite calcitriol (1,25-dihydroxyvitamin D3), which regulates calcium and phosphate homeostasis, bone mineralization, and modulates immune function.
| Onset | Duration | Bioavailability |
|---|---|---|
| Serum levels of 25-hydroxyvitamin D begin to rise within 24 hours of oral administration, but clinical effects on bone metabolism and symptom relief (e.g., myalgia) may take several weeks to months. | Long-lasting due to storage in adipose tissue and muscle. The half-life of 25-hydroxyvitamin D is approximately 15 days, and effects can persist for weeks after discontinuation. | Approximately 55-99% after oral administration, depending on formulation (oil-based softgels have higher bioavailability), dietary fat co-ingestion, and individual gastrointestinal health. |
2. Mechanism of Action
Vitamin D3 is a prohormone. It is converted to 25-hydroxyvitamin D3 in the liver and then to the active hormonal form, 1,25-dihydroxyvitamin D3 (calcitriol), in the kidneys. Calcitriol 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, regulating their transcription.
3. Indications & Uses
- Prevention and treatment of Vitamin D deficiency
- Osteoporosis prevention and treatment (in combination with calcium)
- Nutritional rickets and osteomalacia
- Hypocalcemia associated with hypoparathyroidism (as adjunct)
4. Dosage & Administration
Adult Dosage: **Treatment of Deficiency:** 800-2000 IU daily, or 60,000 IU weekly for 8-12 weeks, followed by maintenance. **Maintenance/Prevention:** 800-2000 IU daily. Indian guidelines (ICMR/ESI) often recommend 800-1000 IU/day for maintenance in at-risk adults.
Administration: Take orally, preferably with the largest meal of the day (containing fats/oils) to enhance absorption. Can be taken at any time of day. Do not crush or chew sustained-release formulations. Maintain adequate dietary calcium intake (~1000 mg/day).
5. Side Effects
Common side effects may include:
- Generally well-tolerated at 800 IU dose. Rarely: constipation, dry mouth, metallic taste, nausea.
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 (e.g., Phenytoin, Phenobarbital, Carbamazepine) | Increase hepatic metabolism of vitamin D to inactive compounds, reducing its efficacy. | Moderate |
| Orlistat, Cholestyramine | Reduce absorption of fat-soluble vitamin D. | Moderate |
| Cardiac Glycosides (Digoxin) | Hypercalcemia may potentiate digoxin toxicity and risk of arrhythmias. | Major |
| CYP3A4 Inducers (e.g., Rifampin) | May increase metabolism of vitamin D metabolites. | Moderate |
7. Patient Counselling
- DO take with your main meal containing fat (e.g., lunch/dinner).
- DO inform your doctor if you are taking any other vitamins, calcium supplements, or medications.
- DO maintain adequate dietary calcium intake (milk, yogurt, leafy greens, fortified foods).
- DON'T exceed the recommended dose without medical advice.
- DON'T take simultaneously with orlistat or bile acid sequestrants; separate by at least 2-4 hours.
8. Toxicology & Storage
Overdose: Manifestations of hypervitaminosis D and hypercalcemia: Early: Nausea, vomiting, constipation, anorexia, weakness, weight loss, polydipsia, polyuria. Late: Confusion, psychosis, renal calculi, nephrocalcinosis, hypertension, cardiac arrhythmias, metastatic calcification.
Storage: Store below 30°C, in a cool, dry place. Protect from light and moisture. Keep out of reach of children. Do not freeze. Store in the original packaging.