Objective To assess the effects of low-dose esketamine on the median effective dose(ED50)of ciprofol for anesthesia induction in painless gastrointestinal endoscopy.Methods Fifty-nine pa-tients underwent elective painless gastrointestinal endoscopy,26 males and 33 females,aged 18-64 years,BMI 18-30 kg/m2,ASA physical status Ⅰ or Ⅱ,were divided into two groups by random number table method:esketamine combined with ciprofol group(group EC,n = 29)and ciprofol group(group C,n = 30).Group EC received intravenous injection of esketamine 0.3 mg/kg and group C received the same amount of normal saline 2 minutes before administration of ciprofol.The initial anesthesia induction dose of ciprofol was 0.4 mg/kg.If a positive reaction occurs during the examination,the next patient will receive an increase dose of propofol 0.04 mg/kg,otherwise will decrease by propofol 0.04 mg/kg.The positive reaction was defined that the patient's BIS can not be decreased to 60 2 minutes after anesthesia induction,or the cough or body movement reaction occur at level 2 or above when entering the mirror.The dosage of ciprofol,recovery time,discharge time,the occurrence of intraoperative and postoperative adverse reactions were recorded.The ED50,95%effective dose(ED95)and 95%confidence interval(CI)of the two groups were calculated by Probit probability regression analysis.Results Compared with group C,the dosage of ciprofol,the incidence of hypotension and frequency of administration of vasoactive drugs during the exami-nation process in group EC were significantly reduced(P＜0.05).The ED50 of ciprofol for anesthesia in-duction in painless gastrointestinal endoscopy in group EC was 0.21 mg/kg(95%CI 0.12-0.25 mg/kg)and the ED95 was 0.32 mg/kg(95%CI 0.26-0.39 mg/kg).The ED50 of ciprofol for anesthesia induction in painless gastrointestinal endoscopy in group C was 0.37 mg/kg(95%CI 0.32-0.40 mg/kg)and the ED95 was 0.48 mg/kg(95%CI 0.43-0.54 mg/kg).The ED50 and ED95 of ciprofol for anesthesia induction in painless gastrointestinal endoscopy in group EC was significantly lower than that in group C(P＜0.05).There was no significant difference in other frequency of adverse events between the two groups.Conclusion Esketamine 0.3 mg/kg can reduce the ED50 of ciprofol in painless gastrointestinal endoscopy and reduce the dosage of ciprofol during the examination process,which is safe for painless gastrointestinal endoscopy with stable intraoperative circulation.