Oral Ondansetron to Improve Patient Experience of Unsedated Esophagogastroduodenoscopy Pilot Study

The goal of this interventional pilot study is to explore the feasibility of administering ondansetron prior to esophagogastroduodenoscopy (EGD), and preliminarily test if it can alleviate the gagging reflex, nausea and vomiting in Chinese patients undergoing unsedated diagnostic EGD. The main questions it aims to answer are: Is adding oral ondansetron to pre-medication regimen of unsedated EGD safe, easy to conduct, and well-accepted by patients? May taking ondansetron prior to EGD reduce the discomfort of patients during the endoscopic process? May taking ondansetron prior to EGD reduce the intensity of patient gagging and nausea, and the incidence of vomiting ? Researchers will compare premedication of oral ondasetron and topical pharyngeal anesthesia (dyclonine hydrochloride mucilage) with topical pharyngeal anesthesia only to answer the questions above. Participants will be treated with one of the following regimens according to the randomization result: * Oral ondansetron of 8 mg 2 hours prior to endoscopic process and dyclonine hydrochloride mucilage to be gurgled 15 minutes prior to endoscopic process; * dyclonine hydrochloride mucilage to be gurgled 15 minutes prior to endoscopic process. After the endoscopic process is finished, patients will be asked to fill in a questionnaire that contains the following items, all measured by NRS 0-10: * Overall discomfort; * Intensity of gagging; * Willingness to undergo unsedated EGD again if indicated.