Maintenance of optimised hair growth from viable terminal scalp hair follicles at baseline with oral finasteride in male pattern hair loss and first evidence of a “drug dependency” and a post‐finasteride “rebound effect”

Background Analytical measures for the pharmacodynamics understanding of drug induced scalp hair responses are lacking. Materials and methods The present study measured in detail dynamics of hair productivity on two scalp test sites showing male pattern hair loss. The natural regression decay rate established after 2 years without treatment was followed by treatment with daily oral intake of 1 mg finasteride. Results While terminal hair (diameter ≥40 μm) were maintained “on‐drug,” within 30 months “off‐drug” MPHL significantly worsened as 94% terminal hair miniaturised and became unproductive. Accordingly, the viable drug responding follicles were qualified as “finasteride dependent” unravelling a hereto unreported “rebound effect” after interruption of the drug intake. Interestingly, the transformation of terminal hair into miniaturised hair occurred only after 12 months without treatment, that is the time necessary to complete a clinically significant full hair cycle initiated during the drug intake. This explains why exogen hair release and miniaturisation occurred only between 12 and 30 months “off‐drug” while resistant hair grew also slower. Conclusion Drug dependency and rebound phenomenon are new findings along with evidence against the hypothesis claiming that terminal hair growth arises from initial vellus hair follicles in drug‐treated MPHL.