A Diuretic is any substance that promotes diuresis, the increased production of urine. This includes forced diuresis. A diuretic tablet is sometimes colloquially called a water tablet. There are several categories of diuretics. All diuretics increase the excretion of water from the body, through the kidneys. They are designed to increase the amount of water and salt expelled from the body as urine. They are often prescribed to help treat high blood pressure, but they are used for other conditions as well.