Diabetes mellitus, often abbreviated as "DM", is a chronic disease in which a deficiency of the hormone insulin impairs the body's ability to metabolise sugar. It is one of the most common endocrine (hormonal) disorders.

There are two types of diabetes in dogs and cats: Type I and Type II. Type I diabetes occurs when the body is unable to produce enough insulin. This can be the result of the destruction of the cells of the pancreas that produce insulin. Dogs with Type I diabetes usually need insulin injections to control the disease. Type II diabetes, on the other hand, occurs when the body is able to produce enough insulin but certain factors interfere with the body's ability to use insulin.

Diabetes most often occurs in animals from middle age to old age (7-9 years) and females are the most prevalent group (females are twice as likely to develop the disease as males). Juvenile diabetes can occur in animals less than 1 year old. Any breed can develop diabetes.

The incidence of diabetes in cats is lower than in dogs. However, when a cat has diabetes, it is more difficult to control than dog diabetes. Obese animals are susceptible to the disease, but those that are not obese are not immune.

Common signs of diabetes are increased thirst and urination, increased appetite and weight loss, which may be masked in cats with other conditions. Other conditions that may occur include vomiting, diarrhoea, loss of appetite, difficulty breathing, weakness and straining to pass urine, which may appear suddenly or last for days or months.