Vitamin D

Trade Names
Ergocalciferol (vitamin D2): Many OTC preparations
Cholecalciferol (vitamin D3): Many OTC preparations
Calcitriol (1,25-dihydroxycholecalciferol): Rocaltrol

Synonyms: Ergocalciferol – vitamin D2; cholecalciferol – vitamin D3;  calcitriol – 1,25-dihydroxycholecalciferol

Drug Class: Vitamin

Ergocalciferol: Capsule 50,000 units (1.25 mg), multiple OTC preparations containing 400 U/tablet
Cholecalciferol: Capsule 1000-, 5000-, 50,000 units
Calcitriol: 0.25-, 0.5 mcg capsules

Ergocalciferol and cholecalciferol: As a dietary supplement/prevention of osteoporosis: 400-800 U/day (to ensure total daily intake of at  least 800 U); For vitamin D  deficiency:   800-4,000 U/day or 50,000 units once a month for 6 months.
Calcitriol: For hypocalcemia in patients on chronic dialysis individualize dose to maintain serum calcium level with usual starting dose of 0.25 mcg daily or alternate days.

Indications: Erocalciferol and cholecalciferol: Most often used as a dietary supplement with calcium to ensure adequate intake of calcium and vitamin D for the prophylaxis and treatment of osteoporosis. Also used to treat rickets/osteomalacia, and vitamin D deficiency. Calcitriol is used to treat hypocalcemia associated with chronic dialysis and renal failure.

Mechanism of Action: Promotes absorption of calcium from GI tract and stimulates calcium reabsorption from the renal tubule

Contraindications: Hypercalcemia, vitamin D toxicity

Precautions: Adequate calcium intake required; avoid hypercalcemia.

Monitoring: With dietary supplementation using doses in the range of the RDA (600 U/day for adults less than 70 and 800 U/day if older than 70) of vitamin D2, serum calcium may be monitored occasionally. With higher doses of vitamin D or use of the more potent calcitriol preparation, close monitoring of serum calcium level is prudent.

Pregnancy Risk: D for doses above the RDA

Adverse Effects: Doses of vitamin D2 in the RDA range have minimal side effects. The more potent calcitriol preparation has more often been associated with hypercalcemia, hypercalciuria, and renal stones.
With any vitamin D preparation, dose-related hypercalcemia may occur, resulting in weakness, anorexia, polyuria, thirst, constipation, nausea, myalgia, irritability, and psychosis.

Drug Interactions: Increased serum calcium with thiazide diuretics

Comments: The effect of vitamin D preparations on osteoporosis is controversial. Generally, vitamin D is supplemented in the treatment and prophylaxis of osteoporosis in doses that ensure a daily intake that meets the RDA. The definition of vitamin D deficiency is similarly controversial. The Institute of Medicine report suggests that the effect of vitamin D plateaus at serum levels of 25-hydroxyvitamin D (25OHD) between 12 and 16 ng/ml in almost all people and that there is little or no additional benefit to levels above 20 ng/ml.  A dose ranging study in women with vitamin D insufficiency found that a vitamin D3 dose of 800 IU/day increased serum 25OHD levels to greater than 50 nmol/L (20 ng/ml) in 97.5% of women.

Clinical Pharmacology: Cholecalciferol (vitamin D3) and ergocalciferol (vitamin D2) are activated in the liver to calcifediol (25-hydroxycholecalciferol) and then in the kidneys to calcitriol (1,25-dihydroxycholecalciferol). Vitamin D is fat soluble and is stored in the liver and fat; thus, effects are prolonged.

Adapted from: RheumaKnowledgy