Adults

For severe Vitamin D deficiency (serum 25(OH)D < 25 nmol/L or 10 ng/mL), a common regimen is 40,000 IU once weekly for 7-10 weeks, followed by maintenance therapy. Exact dosage should be determined by a physician based on deficiency severity and patient response.

Elderly

Same as adults, but monitoring for hypercalcemia might be more critical.

Renal_impairment

Use with caution. High doses may be contra-indicated in severe renal impairment without monitoring due to risk of hyperphosphatemia and hypercalcemia. Active vitamin D metabolites might be preferred.

Onset

Pharmacological effects are not immediate; require conversion to active form. Onset of clinical effect for deficiency correction typically takes weeks.

Excretion

Excreted primarily in bile, with some urinary excretion.

Half life

The half-life of 25-hydroxyvitamin D3 is approximately 2-3 weeks; the half-life of 1,25-dihydroxyvitamin D3 (active form) is about 4-6 hours.

Absorption

Readily absorbed from the small intestine, primarily in the presence of bile salts and dietary fat.

Metabolism

Metabolized in the liver (to 25-hydroxyvitamin D3) and kidneys (to 1,25-dihydroxyvitamin D3).

Corticosteroids

May reduce Vitamin D effects.

Thiazide diuretics

May increase the risk of hypercalcemia.

Phenytoin, Barbiturates

May accelerate Vitamin D metabolism, reducing its effects.

Orlistat, Cholestyramine

May impair absorption of Vitamin D.

Cardiac glycosides (e.g., Digoxin)

Hypercalcemia due to Vitamin D may potentiate digitalis toxicity.