Doxercalciferol, the active ingredient in Hectorol^Ò is a synthetic vitamin D₂ analog that undergoes metabolic activation in vivo to form 1a,25-dihydroxyvitamin D₂ (1a,25-(OH)₂D₂), a naturally occurring, biologically active form of vitamin D₂. Hectorol^Ò is available as soft gelatin capsules containing 0.5 mcg or 2.5 mcg doxercalciferol. Each capsule also contains fractionated triglyceride of coconut oil, ethanol, and butylated hydroxyanisole (BHA). The capsule shells contain gelatin, glycerin, titanium dioxide, and D&C Yellow No. 10 with or without FD&C Red No. 40.

Doxercalciferol is a colorless crystalline compound with a calculated molecular weight of 412.66 and a molecular formula of C₂₈H₄₄O₂. It is soluble in oils and organic solvents, but is relatively insoluble in water. Chemically, doxercalciferol is (1a,3b,5Z,7E,22E)-9,10-seco-ergosta-5,7,10(19),22-tetraene-1,3-diol and has the following structural formula:

Other names frequently used for doxercalciferol are 1a-hydroxyvitamin D₂, 1a-OH-D₂, and 1a-hydroxyergocalciferol.

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