PROPECIA(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. Its structural formula is:

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.

PROPECIA tablets for oral administration are film-coated tablets
that contain 1 mg of finasteride and the following inactive
ingredients: lactose monohydrate, microcrystalline cellulose, pregelatinized starch, sodium starch glycolate, hydroxypropyl methylcellulose, hydroxypropyl cellulose LF, titanium dioxide, magnesium stearate, talc, docusate sodium, yellow ferric oxide, and red ferric oxide.