Off-label
Prescribing an approved drug for a use not listed on its official label, for example a different cancer or an earlier stage. Legal and common in oncology, but insurers may refuse to pay and the evidence is often thinner.
A drug's label (its licence) specifies the diseases, stages, biomarkers and prior treatments for which regulators have judged the evidence sufficient; anything else is off-label. In oncology perhaps a third of prescribing is off-label, driven by tumour-agnostic biology (a KRAS G12C inhibitor approved in lung cancer used in colorectal cancer), by rare cancers where trials are infeasible, and by guideline recommendations that run ahead of regulators. Compendia such as NCCN largely determine which off-label uses insurers will reimburse in the United States, and tumour-agnostic approvals and basket trials are ways of bringing rational off-label use onto the label.