(Rosmarinus officinalis) The virtues of rosemary are extolled by many ancient herbalists, Greek, Roman, Arab, and European. Rosemary has long been used to prevent balding and as a hair conditioner. Scientific research has shown rosemary leaf extract to inhibit 5-alpha-reductase, the enzyme that converts testosterone into a more potent androgen dihydrotestosterone (DHT). It is DHT that shrinks or miniaturizes the hair follicle, which eventually leads to baldness. Preliminary clinical studies have shown that applying  rosemary oil to the scalp improves hair growth and is comparable to minoxidil.*

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Promotion of hair growth by Rosmarinus officinalis leaf extract. (United States). Murata, Kazuya; Noguchi, Kazuma; Kondo, Masato; Onishi, Mariko; Watanabe, Naoko; Okamura, Katsumasa; Matsuda, Hideaki, 2013-02-01