The object of the present invention is the use of compounds designed to inhibit the activity of the enzyme 5α-reductase. This is a novel use of compounds of general formula (I): CH3(—CH═CH)n—R  (I) where n=from 2 to 7, and R is chosen from among: CHO, CH2OH, CH2O—CO—R′, CO—O(−), where R′ is chosen from among H, and the alky from C1 to C22, each compound of general formula (I) being used as such or in mixtures with other compounds, as the active principle in a pharmaceutical or cosmetic composition designed to inhibit the action of the enzyme 5α-reductase, and of the pharmaceutical or cosmetic compositions deriving therefrom.