Postoperative nausea and vomiting (PONV) are frequent adverse effects of surgery and anesthesia. Dexamethasone is well established for the prophylaxis of PONV, but the efficacy of dexamethasone for the treatment of established PONV symptoms remains unknown. The primary objective of the main study is to test the antiemetic efficacy of dexamethasone for the treatment of established PONV in adults undergoing surgery under general anaesthesia, and to test for dose-responsiveness. The secondary objective is the evaluation of the potential adverse effect profile of dexamethasone, No prophylactic antiemetics are allowed. Premedication, conduct of anesthesia and postoperative analgesia will be at the discretion of the responsible anesthesiologist. Patients who have received their assigned study drug and who continue to experience PONV will receive antiemetic rescue treatment (ondansetron, droperidol). The minimum delay between administration of the study drug and rescue is 60 min (to ensure that dexamethasone has the scope to show antiemetic efficacy). Primary efficacy endpoint: Complete absence of any nausea and/or vomiting (including retching) in a previously nauseated or vomiting patient within 24 hours after administration of the study treatment. Secondary endpoints: Time to treatment failure; quality of sleep during the first postoperative night (numerical rating scale ranging from 0 = no sleep at all to 10 = excellent sleep); blood glucose (in the morning after administration of study drug); any minor or major adverse effects during 24h.