Drugs such as steroids, Dilantin, and others may elevate the blood sugar through a variety of mechanisms. Certain other drugs, such as alloxan, streptozocin, and thiazide diuretics, are toxic to the beta cells of the pancreas and can cause diabetes. Certain syndromes (for example, Prader-Willi, Down's, Progeria, and Turner's) may result in a hyperglycemic state; if this state is prolonged, the result can be permanent diabetes.

Diabetes resulting in an insulin-dependent state is classified as Type 1 diabetes. While Type 1 diabetes affects only between 5 to10 percent of the diabetic population, its effects on the body can be worse than other forms of diabetes. In the past, Type 1 has been known as juvenile or juvenile-onset diabetes (because it is usually diagnosed in those under thirty), brittle diabetes, unstable diabetes, and ketosis-prone diabetes. People in this classification more frequently exhibit the classic symptoms, usually with ketones present in blood and urine. A blood-sugar level of 800 mg/dl (44 mmol) or more, especially if ketones are not present, indicates a diagnosis of hyperglycemic hyperosmolar nonketotic syndrome (a state in which the body is extremely dry (dehydrated), the chemicals in the body are concentrated, and the blood sugar is high).

As stated before, diabetes is a syndrome or group of diseases (rather than one disease), leading to the prolonged hyperglycemic state. Type 1 is most associated with the killing of the beta cells, most likely by the body's own immune system. Either the immune system cannot kill an infecting agent, which then kills the beta cells, or the immune system itself goes "wild," attacking the body's own tissue and destroying the beta cells. The cells of the islets of Langerhans are inflamed, resulting from an infectious-disease process (for example, mumps) or, more commonly, from an autoimmune (allergic to self) response.

The autoimmune process results in the circulation of antibodies that may either cause or be caused by beta-cell death. If it is found that the antibodies cause beta-cell destruction (the body fighting what it now considers foreign to itself), the body's response to the Type 1 diabetes is much less severe (i.e., easier to control) with treatment. Until then, the outcome is a lack of available insulin. While the onset is said to be sudden, changes resulting in decreased insulin availability may have occurred over a longer period of time. In short, insulin-dependent diabetes mellitus is an inherited defect of the body's immune system, resulting in destruction of the insulin-producing beta cells of the pancreas.

Heredity is a major cause of diabetes. If both parents have Type 2 diabetes, there is a chance that nearly all of their children will have diabetes. If both parents have Type 1 diabetes, fewer than 20 percent of their children will develop Type 1 diabetes. In identical twins, if one twin develops Type 2 diabetes, the chance is nearly 100 percent that the other twin will also develop it. In Type 1 diabetes, however, only 40 to 50 percent of the second twins will develop the disease, indicating that while inheritance is important, environmental factors (for example, too much food, too much stress, viral infection, and so forth) are also involved in the development of Type 1 diabetes.