How does it work Dihydrotestosterone (DHT) is a male sex hormone which in high levels is thought to contribute to male pattern baldness. DHT shrinks the hair follicles resulting in decreased growth and thinner hairs. It is produced by an enzyme called 5-alpha reductase in the hair follicles. Men who have less of this enzyme have lower levels of DHT and are less likely to develop male pattern baldness. Propecia has been shown to inhibit the human Type II 5-alpha reductase, decreasing the formation of DHT from testosterone and thus helping to inhibit hair loss. Can I take Propecia? Propecia is suitable for men aged 18 and above. If you have a prostate condition or if your partner is pregnant or trying to get pregnant you cannot take Propecia. How do I take Propecia? Propecia is taken once a day. It may take 3-6 months before you start to see the full effects of Propecia and if you stop taking Propecia you may lose the hair you have gained within 9-12 months.