They are worn by those who are ambulatory in most cases, helping calf muscles to perform their pumping action more efficiently to return blood to the heart. In some cases, they are worn by those at increased risk of circulatory problems, such as diabetics, whose legs are prone to excessive swelling. A common indicator for the prescription of such stockings is chronic peripheral venous insufficiency, caused by incompetent perforator veins. Low-pressure compression stockings are available without prescription in most countries, and may be purchased at a pharmacy or medical supply store. Stockings with a higher pressure gradient, say, above 25–30 mmHg, may require a prescription from a doctor.