Inclusion Criteria:  ≥ 18 years of age Acute moderate to severe pain defined by self-reporting pain ≥4 on NRS Capable of giving informed consent Normal physiology  Exclusion Criteria:  Life-threatening or limb-threatening condition requiring immediate management Pregnancy or breastfeeding Know allergies, hypersensitivity or serious side effects to opioids or methoxyflurane or other excipients Head injury or medical conditions with neurological impairment (Glasgow Coma Scale (GCS)<14) Previous malignant hyperthermia or persons with suspect genetic predisposition for malignant hyperthermia Massive facial trauma, visible nasal blockage or on-going nose bleeding History of severe liver disease with jaundice and scleral icterus Dialysis or history of severe renal disease (known chronic kidney failure stage 4 or 5) Mono Amine Oxidase-inhibitors last 14 days (pharmacological treatment of depression, Mb Parkinson or narcolepsy) Myasthenia gravis Use of investigational medicinal product (IMP) analgesics 12 hours prior to inclusion Any condition that in the view of the study worker would suggest that the patient is unable to comply with study protocol and procedures.