Potassium-channel openers activate potassium channels in vascular smooth muscle, causing relaxation and vasodilation. They are particularly effective at dilating small arteries and arterioles, reducing systemic vascular resistance and blood pressure. This leads to baroreceptor-mediated tachycardia as blood pressure falls. Potassium-channel openers are used to treat refractory or severe hypertension, often in conjunction with beta-blockers and diuretics. The only potassium-channel opener approved for human use is minoxidil, which commonly causes side effects like headaches, flushing, tachycardia, and fluid retention.