Study	Reason for exclusion
Antonini 1994	Not a RCT.
Atanassoff 1994	Not general anaesthesia.
Atassi 2005	Nitrous oxide used in the control group.
Barr 1999	Not a RCT.
Bronco 2010	Nitrous oxide used in the control group.
Castéra 2001	Not general anaesthesia.
Cheong 2000	Nitrous oxide used in the control group.
Divatia 1996	Not a RCT.
Dover 1994	Not a RCT.
Einarsson 1997	Nitrous oxide used in the control group.
Fredman 1998	Nitrous oxide used in the control group.
Gozdemir 2007	Nitrous oxide used in the control group.
Haessler 1993	Nitrous oxide used in the control group.
Haraguchi 1995	Not general anaesthesia.
Heath 1996	Not general anaesthesia.
Holst 1993	Nitrous oxide used in the control group.
Ishii 1994	Nitrous oxide used in the control group.
Jastak 1973	Non‐adults involvement.
Jellish 1996	Nitrous oxide used in the control group.
Johnson 1997	Non‐adults involvement.
Kryshtalskyj 1990	Not general anaesthesia.
Lim 1992	Non‐adults involvement.
Losasso 1992	Non‐adults involvement.
Masood 2002	Not general anaesthesia.
Morimoto 1997	Not a RCT.
Nightingale 1992	Non‐adults involvement.
Nishiyama 1998	Nitrous oxide used in the control group.
Ogg 1983	Non‐adults involvement.
Rocca 2000	Non‐adults involvement.
Saïssy 2000	Non‐adults involvement.
Simpson 1977	Nitrous oxide used in the control group.
Sinha 2006	Nitrous oxide used in the control group.
Smith 1993	Nitrous oxide used in the control group.
Taki 2003	Non‐adults involvement.
Towey 1979	Non‐adults involvement.
Van den Berg 1995	Non‐adults involvement.
Vari 2010	Nitrous oxide used in the control group.
Wesner 2005	Not a RCT.
Yamakage 2001	Nitrous oxide used in the control group.
Yang 2004	Nitrous oxide used in the control group.
Zuurmond 1986	Nitrous oxide used in the control group.