All obtainable investigations that have compared the incidence of vomiting in groups of patients who received nitrous oxide (N₂ O) and in patients who received anesthetics or analgesics without N₂ O were examined for a single, dichotomous variable:whether patients who received N₂ O experienced an absolutely higher incidence, as distinct from a statistically significantly higher incidence, of vomiting. The null hypothesis is that N₂ O has no effect on emesis, such that an increased incidence of vomiting should occur in about half of the studies examined. However, patients receiving N₂ O experienced an absolutely higher incidence of emesis in 24 of 27 investigations. The two-tailed probability that this result occurred by chance is <0.00005. It follows that N₂ O increases the incidence of emesis compared to alternative anesthetics.

(Anesth Analg 1996;83:114-6)