Adults

For severe deficiency: 40,000 IU once weekly for 7-10 weeks, followed by maintenance dose. For moderate deficiency: 40,000 IU once every 2-4 weeks. Maintenance dose: 800-2000 IU daily (or equivalent weekly/monthly doses).

Elderly

Same as adults, but monitoring may be more frequent due to potential for impaired renal function and increased risk of falls. Higher incidence of deficiency.

Renal_impairment

Use with caution in patients with severe renal impairment. Regular monitoring of calcium and phosphate levels is crucial. Active vitamin D metabolites may be preferred in severe cases.

Onset

Therapeutic effects may take several days to weeks to manifest fully as Vitamin D stores are replenished and active metabolite levels rise.

Excretion

Excreted primarily in the bile and feces, with a small amount excreted in the urine.

Half life

The half-life of cholecalciferol in the circulation is approximately 24 hours, but its active metabolite, calcitriol, has a half-life of about 3-6 hours. However, the fat-soluble nature means it can be stored in adipose tissue for extended periods, leading to a much longer effective biological half-life of weeks to months.

Absorption

Cholecalciferol is readily absorbed from the gastrointestinal tract, primarily in the small intestine. Its absorption is enhanced by the presence of bile salts and dietary fat.

Metabolism

Metabolized in the liver by 25-hydroxylase to 25-hydroxyvitamin D (calcifediol) and then in the kidneys by 1-alpha-hydroxylase to 1,25-dihydroxyvitamin D (calcitriol), the active form.

Corticosteroids

May reduce the effects of vitamin D by impairing its metabolism and absorption.

Thiazide diuretics

May increase the risk of hypercalcemia by decreasing urinary calcium excretion.

Antacids containing magnesium

Concurrent use with vitamin D may lead to hypermagnesemia, especially in patients with renal impairment.

Seizure medications (e.g., phenytoin, barbiturates)

May increase vitamin D metabolism, potentially leading to vitamin D deficiency.