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Entonox® is a 50:50 mixture of nitrous oxide and oxygen. It is an effective analgesic agent with rapid onset and offset characteristics. Its effects are predictable and reliable, with minimal side effects. It is widely used but its mechanism of action has not been fully explained. In large obstetric units the gas is piped, but in smaller units and the community it is supplied in cylinders (blue with a blue and white shoulder). It is self-administered, but the woman needs to be adequately informed of the functioning of the apparatus. Optimal analgesia is obtained by closely applying the lips around the mouthpiece: the gases take affect within 20 seconds, so it is important that the woman uses it before a contraction. Maximum efficacy is achieved after 45-50 seconds, and if correctly timed, this should coincide with the height of the contraction. The use of scavenging equipment, to extract expired gases, is recommended for all birthing rooms.

Further reading

BOC (2015). Entonox® - The essential guide. Available at:


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