Potassium chloride is a mineral supplement used to treat or prevent low levels of potassium in the blood.The supplement, often referred to as the formula KCl, is available from dozens of manufacturers. Potassium is essential for the function of the heart, muscles, kidneys, nerves, and digestive system.he supplement, often referred to as the formula KCl, is available from dozens of manufacturers. Potassium is essential for the function of the heart, muscles, kidneys, nerves, and digestive system.