Minoxidil was originally developed as a blood pressure medication; once its hair-growth side effect was noticed, it was reformulated topically (and later as low-dose oral) for hair loss.
Mechanism of Action
The exact mechanism is still being studied, but it's believed to increase blood flow to follicles and modulate growth factors in ways that prolong the anagen phase and encourage follicles to shift to a larger diameter.
Use and Efficacy
Topical 5% solution/foam is applied once or twice daily. Low-dose oral minoxidil (0.25-5 mg/day) is increasingly favored, especially for patients who don't respond to or struggle with the topical form; in efficacy rankings it falls behind dutasteride and finasteride.
Side Effects
The most common topical side effects are contact dermatitis and unwanted hair growth elsewhere (hypertrichosis). Oral use can cause dose-dependent hypertrichosis and cardiovascular effects (palpitations, fluid retention).