Few studies into conscious sedation with 50% nitrous oxide/oxygen premix (50% N2O/O2) have been conducted in accordance with Good Clinical Practice in Clinical Trials. Of the 140 articles retained in this review, the incidence of adverse events (AEs) varied in the range of 0-68% according to the indications. When other drugs and/or local anaesthesia were used concomitantly, the relative risk for nausea and vomiting increased whereas it decreased for vertigo or hallucinations. Only one study examined potential causal relationship between serious AEs and the 50% N2O/O2 gas itself, giving a risk for a serious AE directly due to the gas of 3/10,000 administrations. No analysis of the effect of the method of administration was possible, nor could conclusions be drawn relating to the risk of occupational exposure.