A diuretic is any substance that promotes diuresis, that is, the increased production of urine. This includes forced diuresis. There are several categories of diuretics. All diuretics increase the excretion of water from bodies, although each class does so in a distinct way. Alternatively, an antidiuretic such as vasopressin, or antidiuretic hormone, is an agent or drug which reduces the excretion of water in urine.