Drugs used in diabetes treat diabetes mellitus by altering the glucose level in the blood. With the exceptions of insulin, exenatide, liraglutide and pramlintide, all are administered orally and are thus also called oral hypoglycemic agents or oral antihyperglycemic agents. There are different classes of anti-diabetic drugs, and their selection depends on the nature of the diabetes, age and situation of the person, as well as other factors.

Diabetes mellitus type 1 is a disease caused by the lack of insulin. Insulin must be used in type 1, which must be injected.

Diabetes mellitus type 2 is a disease of insulin resistance by cells. Type 2 diabetes mellitus is the most common type of diabetes. Treatments include agents that increase the amount of insulin secreted by the pancreas, increase the sensitivity of target organs to insulin, decrease the rate at which glucose is absorbed from the gastrointestinal tract, and increase loss of glucose through urination.

Several groups of drugs, mostly given by mouth, are effective in type 2, often in combination. The therapeutic combination in type 2 may include insulin, not necessarily because oral agents have failed completely, but in search of a desired combination of effects. The great advantage of injected insulin in type 2 is that a well-educated patient can adjust the dose, or even take additional doses, when blood glucose levels measured by the patient, usually with a simple meter, as needed by the measured amount of sugar in the blood.

Insulin is usually given subcutaneously, either by injections or by an insulin pump. In acute care settings, insulin may also be given intravenously. Insulins are typically characterized by the rate at which they are metabolized by the body, yielding different peak times and durations of action. Faster-acting insulins peak quickly and are subsequently metabolized, while longer-acting insulins tend to have extended peak times and remain active in the body for more significant periods.

Insulin degludec is sometimes classed separately as an "ultra-long" acting insulin due to its duration of action of about 42 hours, compared with 24 hours for most other long acting insulin preparations.

Most oral anti-diabetic agents are contraindicated in pregnancy, in which insulin is preferred.

Insulin is not administered by other routes, although this has been studied. An inhaled form was briefly licensed, but subsequently withdrawn.