Minoxidil
In plain terms
The active drug in over-the-counter hair-regrowth products, applied to the scalp as a liquid or foam. It widens small blood vessels and lengthens the hair-growth phase, which is how it regrows thinning hair.
What the evidence shows
Minoxidil is an FDA-approved hair-regrowth drug with a long track record, and its cautions come from the fact that it is a repurposed blood-pressure medicine. Applied to the scalp, only a small fraction, on the order of one to two percent of the dose, reaches the bloodstream, so most people use it without a body-wide effect. When enough does get absorbed it can act as the vasodilator it is, with a faster heartbeat, lightheadedness, or fluid retention, which is why the label tells anyone with heart disease to check with a doctor first and to stop for chest pain or a racing heart. Two other things come with it. The product is not for use in pregnancy: the label advises against it, and animal studies showed reduced fertility and fewer live pups in one model, so the safe move is to set it aside while pregnant or nursing. And it commonly causes an early burst of shedding in the first weeks as hairs cycle, along with scalp irritation that often traces to the propylene glycol in the liquid rather than the drug. Used as directed by a healthy adult it is well tolerated; the reasons it lands at depends are the heart caution, pregnancy, and the local irritation.
On the label
Listed as minoxidil among the active ingredients, at 2 or 5 percent. Check with a doctor first if you have heart trouble, skip it in pregnancy, and expect some early shedding; if the liquid irritates your scalp, a propylene-glycol-free foam often helps.