Off-label use is the practice of prescribing drugs for a purpose outside the scope of the drug's approved label, most often concerning the drug's indication. In the United States, the Food and Drug Administration (FDA) requires numerous clinical trials to prove a drug's safety and efficacy in treating a given disease or condition. If satisfied that the drug is safe and effective, the drug's manufacturer and the FDA agree on specific language describing dosage, route and other information to be included on the drug's label. More detail is included in the drug's package insert. However, once the FDA approves a drug for prescription use, they do not attempt to regulate the practice of medicine, and so the physician makes decisions based on her or his best judgment. It is entirely legal in the United States and in many other countries to use drugs off-label. Exceptions to this are certain controlled substances, such as opiates, which cannot be legally prescribed except for approved purposes in the U.S. In Australia, amphetamines are included in these drugs which cannot be prescribed off-label.

Some drugs are used more frequently off-label than for their original, FDA-approved indications. A 1991 study by the U.S. General Accounting Office found that one-third of all drug administrations to cancer patients were off label and more than half of cancer patients received at least one drug for an off-label indication . Frequently the standard of care for a particular type or stage of cancer involves the off-label use of one or more drugs. An example is the use of tricyclic antidepressants to treat neuropathic pain. This old class of antidepressants is now rarely used for clinical depression due to side effects, but the tricyclics are often effective for treating pain.