Entonox (nitrous oxide) is a gaseous mixture of 50% nitrous oxide (N₂O) and 50% oxygen (O₂). It is a patient‐controlled, inhaled analgesic which is used for the short‐term relief of acute pain (BOC [17]). Nitrous oxide is a colourless, sweet‐smelling gas with powerful analgesic properties, supplied in premixed cylinders (Bruce and Franck [27]). The gas is inhaled and self‐administered by the patient using a demand valve system attached to a facemask or mouthpiece. The nitrous oxide component of the gas acts as an analgesic, producing similar physiological effects to opioids (Emmanouil and Quock [64]), whilst the oxygen component has an antihypoxic effect (Faddy and Garlick [66]) and ensures good cerebral perfusion and enhanced recovery (Peate and Lancaster [172]).