Eligible children and adolescents will be randomised in a 1:1 ratio (220 eligible patients; 176 children and 44 adolescents) to have 110 patients per treatment arm (44 children aged 6 to 8 years, 44 children aged 9 to 11 years and 22 adolescents aged 12 to < 18 years). Eligible patients will receive either methoxyflurane or placebo from the PENTHROX® inhaler in a 1:1 ratio.