Finasteride, a synthetic 4-azasteroid compound, is a specific inhibitor of steroid Type II 5a-reductase, an intracellular enzyme that converts the androgen testosterone into 5a-dihydrotestosterone (DHT).

Finasteride is 4-azaandrost-1-ene-17-carboxamide,N-(1,1-dimethylethyl)-3-oxo-,(5a,17b)-. The empirical formula of finasteride is C23H36N2O2 and its molecular weight is 372.55. Finasteride is a white crystalline powder with a melting point near 250°C. It is freely soluble in chloroform and in lower alcohol solvents but is practically insoluble in water.