Twnety-four RCTs involving 2,478 patients were included.
Early and late post-operative nausea: omission of nitrous oxide showed no effect; ORs were 1.3 (95% CI: 0.9, 1.8) and 1.1 (95% CI: 0.9, 1.5), respectively, and NNT 30 and 36.9.
Early and late vomiting: omission of nitrous oxide showed a statistically-significant improvement; ORs were 2.4 (95% CI: 1.7, 3.3) and 1.5 (95% CI: 1.2, 1.8), respectively, and NNT 11.8 and 13.8.
Subgroup analysis based on baseline risk. Early vomiting (16 studies): low baseline risk, OR 1.4 (95% CI: 0.8, 2.4); high baseline risk, OR 3.1 (95% CI: 2.1, 4.6). Late vomiting (12 studies): low baseline risk, OR 1.2 (95% CI: 0.9, 1.6); high baseline risk, OR 2.1 (95% CI:1.5, 2.9).
Intra-operative awareness (7 studies): the NNT for 1 patient experiencing intra-operative awareness with a nitrous oxide-free anaesthetic, compared with a regime with nitrous oxide, was 46.2; the OR was 4.5 (95% CI: 1.1, 18).