Off-label prescribing is the prescription of a registered medicine for a use that is not included in the product information. The practice is common, with rates up to 40% in adults and up to 90% in paediatric patients. Off-label prescribing is not illegal and may sometimes be clinically appropriate, but is associated with a number of clinical, safety and ethical issues. To date, no explicit guidance has been available to help clinicians assess appropriateness in off-label prescribing. We describe the development of a guide for clinicians, policymakers and funders of health care in evaluating the appropriateness of medicines proposed for off-label use. Three broad categories of appropriate off-label use are identified:off-label use justified by high-quality evidence; use within the context of a formal research proposal; and exceptional use, justified by individual clinical circumstances. An appropriate process for informed consent is proposed for each category. If there is no high-quality evidence supporting off-label use, and the medicine is not suitable for exceptional or research indications, its use is generally not recommended. This will reduce inappropriate use, enhance patient safety by reducing exposure to unnecessary risk, and may stimulate more clinically relevant medicines research.