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" ## Abstract 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 sign