Ondansetron Combined with Dyclonine Hydrochloride to Improve Patient Experience in Unsedated Esophagogastro-duodenoscopy

The goal of this clinical trial is to learn if administering ondansetron prior to esophagogastroduodenoscopy (EGD) can alleviate the gagging reflex, nausea and vomiting in Chinese patients undergoing unsedated diagnostic EGD. The main questions it aims to answer are: Does taking ondansetron prior to EGD reduce the discomfort of patients during the endoscopic process? Does 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 see if ondansetron can exert the above-mentioned effects. 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.