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Entonox, referred to colloquially as gas and air, is an analgesic used to ease pain during labour, trauma and minor surgical procedures. Entonox is the trade name for a mixture of fifty percent nitrous oxide (N<sub>2</sub>O or laughing gas) and fifty percent oxygen (O<sub>2</sub>) so a more appropriate name would be gas and oxygen. Entonox is administered by inhalation from a cylinder at a pressure of 137 bar (in the UK) or from a pipeline supply.