What is the mechanism of action of minoxidil in promoting hair growth and what are common topical adverse effects?

• A. It inhibits 5-alpha-reductase and reduces DHT levels.
• B. It blocks androgen receptors on hair follicles.
• C. It decreases blood flow to scalp.
• D. It opens potassium channels to promote vasodilation and prolongs the anagen phase; common adverse effects include scalp irritation and unwanted hair growth outside treated areas.

Answer: (D)

Minoxidil promotes hair growth primarily by opening potassium channels in scalp vascular smooth muscle, which causes vasodilation and increased blood flow to the hair follicles. This improved perfusion helps follicles receive more nutrients and signaling that encourage hair follicles to enter and stay in the growth (anagen) phase, often prolonging that phase and resulting in thicker, longer hair in the treated areas. The common topical adverse effects are scalp irritation and unwanted hair growth outside the treated area due to diffusion or modest systemic absorption. This mechanism differs from agents that reduce DHT or block androgen receptors, and it does not work by decreasing blood flow, which would undermine the vasodilatory effect.