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 (N2O or laughing gas) and fifty percent oxygen (O2) 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.