Category: receptor_pharmacology
Explains the alarming temporary increase in hair shedding that hits 2-8 weeks after starting topical or oral minoxidil — the most common reason users abandon the drug before it has time to work. Minoxidil is a K⁺-ATP channel opener; in vascular smooth muscle this drives the original-indication antihypertensive effect, but in the hair follicle the pharmacology is different (Suchonwanit 2019 review; Messenger 2004 mechanism summary). The active metabolite is minoxidil sulfate, formed in the follicle by SULT1A1 — sulfotransferase activity is the rate-limiting step and is the basis of the 20-40% non-responder phenotype (Goren 2014 / Roberts 2014 showed plucked-hair SULT1A1 activity predicts clinical response). Follicular K⁺-ATP opening and downstream effects (VEGF upregulation, PGE2 increase, β-catenin signaling in the dermal papilla, Buhl 1989) shorten the telogen (resting) phase and trigger premature anagen (growth) entry. The catch: telogen hairs about to fall out anyway are pushed out simultaneously to make room for the synchronized new anagen growth — producing the visible shed at weeks 2-8. The hairs released ARE the old, miniaturized ones that were going to shed eventually; their loss is necessary for replacement. New anagen hairs become visible around weeks 12-16, with full effect at 6-12 months. Counseling matters: users not warned about the shed often interpret it as drug failure and stop — Mysore counseling protocols and ISHRS guidelines now flag this as a key pre-prescription discussion item. Olsen 1986 was the first dose-response demonstration of clinical efficacy in male pattern baldness.