Postoperative nausea and vomiting (PONV) are frequent after surgery and anaesthesia. Dexamethasone is widely used as antiemetic for the prevention of PONV. Little is known about the efficacy of antiemetic drugs for the treatment of established PONV symptoms. No single randomised trial has been published so far that tests the efficacy of dexamethasone for the treatment of established PONV symptoms. In this trial the investigators want to test the antiemetic efficacy of three different doses of intravenous dexamethasone for the treatment of established PONV symptoms. In adjunct protocols of this study the investigators aim to establish a novel method to quantify the anti-nausea efficacy of an antiemetic drug, to study pharmacogenetics of PONV, and to further our understanding on the smoking status as a predictive factor of PONV.