Introduction Nitrous oxide has been used for decades as an inhalational analgesic in labour. It is considered to be generally safe as it is rapidly eliminated via the lungs preventing accumulation in the mother or fetus. In labour its use is intermittent and for a relatively short period of time. This case highlights a potentially serious side effect occurring from incorrect use.

Case A 27 year old lady with a known disc prolapse was admitted with back pain at 30 weeks gestation. Initially she was managed with oral analgesics and subsequently a nerve root injection provided temporary pain relief. Her pain continued to worsen and she requested Entonox as an adjunct to her oral analgesics. An elective caesarean section was planned for 34 weeks gestation. The day prior to her caesarean section her bloods were checked and revealed bone marrow suppression (Hb 96g/L, Platelets 96 × 10⁹/L, WCC 2.6 × 10⁹/L). Her caesarean section was delayed, the Entonox was stopped and her haematology values spontaneously corrected themselves over the next week.

Discussion Bone marrow suppression is a rare, recognised side effect of prolonged use of nitrous oxide. This occurs via inhibition of methionine synthase leading to interference with the formation of methionine and subsequently synthesis of thymidine and DNA. Bone marrow recovery can be accelerated by folinic acid. Although Entonox is a drug we perceive to be relatively safe it is important that it is used correctly to avoid potentially causing significant morbidity.