Certain groups of patients at high risk of Vitamin D deficiency may require higher doses, and blood concentrations of 25-(OH)D should be monitored:
• alcohol abusers
• institutionalized or hospitalized patients
• dark-skinned patients
• patients with hepatobiliary system diseases such as hepatic dysfunction, cirrhosis, obstructive jaundice
• patients with impaired absorption, including malabsorption, inflammatory bowel disease and gluten enteropathy
• patients with insufficient insolation due to protective clothing or constant use of sunscreen
• obese patients
• patients with diagnosed osteoporosis
• patients taking concomitant medications (e.g., anticonvulsants, glucocorticoids)
• patients who underwent Vitamin D deficiency therapy, requiring maintenance therapy
Blood levels of calcium and phosphate should be assessed before starting Vitamin D treatment. Adequate dietary calcium intake should be ensured for treatment to be effective. Patients should take supplemental calcium if intake from food is insufficient.
When adjusting the dose of Vitamin D, its intake from other sources (from food, dietary supplements, and sun exposure) should be taken into account; the dosage should be adjusted on an individual basis.
During Vitamin D therapy, the level of calcium and phosphorus intake is important for the best effect.
In patients taking pharmacologic doses of Vitamin D, plasma calcium concentration should be assessed regularly whenever vomiting occurs.
Individual hypersensitivity to Vitamin D may vary, therefore, its dosage may be adjusted according to clinical efficacy.
In hemodialysis patients, phosphate-binding agents (phosphate binders) may be used to manage elevated plasma phosphate levels. During Vitamin D therapy, it may be necessary to increase the dose of phosphate-binding agents due to improved phosphate absorption. The plasma calcium-phosphate product (Ca×P, mg/dL) should not exceed 60.
In malabsorption or liver disease, Vitamin D deficiency often requires higher doses for treatment, up to 1 mg (40,000 IU) daily. Doses up to 2.5 mg (100,000 IU) may be used to treat hypocalcemia due to hypoparathyroidism.
Drug Interactions
Concomitant use of anticonvulsants (e.g., phenytoin), hydantoin, primidone, or barbiturates (and possibly other agents that cause induction of hepatic enzymes) may reduce the effect of Vitamin D as a result of metabolic inactivation.
Concomitant use of glucocorticoids may reduce the effect of Vitamin D.
Systemic corticosteroids inhibit calcium absorption.
Long-term use of corticosteroids may be compensated by the effects of Vitamin D.
When administered concurrently with digitalis-containing drugs or other cardiac glycosides, Vitamin D may increase the risk of cardiac glycoside toxicity (arrhythmia). Close medical supervision is required, and, if necessary, serum calcium concentration and ECG should be monitored.