Under the influence describes a person whose mental or physical faculties are impaired by alcohol, drugs (including prescription or illegal drugs) or other intoxicants, so that their judgement, perception, coordination or ability to comply with legal duties is affected.In criminal law, the phrase is most commonly associated with driving offences (drink‑driving and drug‑driving). Across England and Wales, Scotland, Northern Ireland and Ireland, road traffic legislation uses more precise concepts such as being “over the prescribed limit” or “unfit to drive through drink or drugs”, and case law addresses what constitutes impairment. “Under the influence” is therefore often a shorthand rather than a statutory term, though it may appear in charge wording, judicial directions or evidential discussion.The concept is also relevant in assessing capacity and consent (for example, in offences against the person or sexual offences), fitness to work or practise (healthcare, aviation, transport, regulated professions), and in employment and disciplinary contexts. Practitioners should consider statutory thresholds, evidential requirements (breath, blood or urine analysis; field impairment tests; expert evidence) and whether impairment, prescribed limits or simple presence of a substance must be proved in the relevant jurisdiction.