Twenty-four of twenty-seven studies show a greater incidence of emesis associated with nitrous oxide than with alternative anesthetics.

All obtainable investigations that have compared the incidence of vomiting in groups of patients who received nitrous oxide (N2O) and in patients who received anesthetics or analgesics without N2O were examined for a single, dichotomous variable: whether patients who received N2O experienced an absolutely higher incidence, as distinct from a statistically significantly higher incidence, of vomiting. The null hypothesis is that N2O has no effect on emesis, such that an increased incidence of vomiting should occur in about half of the studies examined. However, patients receiving N2O 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 N2O increases the incidence of emesis compared to alternative anesthetics.