Ketoconazole is a topical and oral antifungal medication. It is used topically to reduce the presence of fungus that might be triggering dandruff, and orally to reduce systemic fungal infections. It has been observed that oral doses of ketoconazole can lower serum testosterone and thereby reduce the presence of DHT (Sources: Journal of Dermatological
Science, January 2007, pages 66-68; and Hormone and Metabolic Research, August 1992, pages 367-370). However, there are serious side effects to taking ketoconazole orally, including dizziness, nausea, and headaches. Due to the associated problems with taking ketoconazole orally, there are those who think that applying it topically, in the form of the anti-dandruff shampoo Nizoral, can produce the same testosterone-lowering effect as the oral dosage. However, there is no research of any kind demonstrating this to be the case.