The aim of this prospective dose-finding study is to determine the ED50 of propofol required for successful colonoscope insertion in adult patients. The study is designed using Dixon's up-and-down sequential allocation method. Successful colonoscope insertion is defined as the absence of pain, movement, or vocal response, together with a MOAA/S score ≤2 at the second minute after induction, and maintained until the colonoscope passes the rectosigmoid junction. In cases of failure, an additional dose of propofol is administered, and the initial dose for the next patient is increased by 0.1 mg/kg. In cases of success, the initial dose for the next patient is decreased by 0.1 mg/kg.

The primary outcome is the determination of the ED50 of propofol for successful colonoscope insertion until passage of the rectosigmoid junction.

The procedure is performed by an experienced endoscopist. If, during passage of the colonoscope through the rectosigmoid junction, the patient moves, experiences pain, vocalizes, or if the MOAA/S score is ≥3 at the second minute after induction, the insertion is defined as unsuccessful, and an additional rescue dose of 0.25-0.5 mg/kg propofol is titrated intravenously. The initial propofol dose for the next patient is then increased by 0.1 mg/kg.

If there is no movement during passage of the rectosigmoid junction, the patient reports no pain, and the MOAA/S score is ≤2 at the second minute after induction, the insertion is defined as successful, and the initial dose for the next patient is decreased by 0.1 mg/kg.

The conditions for colonoscope insertion are evaluated only at the initial attempt. If, after the start of the procedure, the patient experiences pain, moves, vocalizes, or the MOAA/S score is ≥3, an additional dose of 0.25-0.5 mg/kg propofol is titrated as required.

Throughout the procedure, the target is to maintain a MOAA/S score ≤2, which is systematically assessed at 2-minute intervals. Vital signs, procedure duration, and drug dosages are recorded for all patients.

At the end of the procedure, patients are awakened with verbal and tactile stimuli. After completion of colonoscopy, all patients are transferred to the post-anesthesia care unit for close monitoring until achieving a Modified Aldrete score of 10. Patients with a Modified Aldrete score of 10 are discharged.

Inclusion Criteria:

• Patients who provided informed consent to participate
• Patients scheduled for colonoscopy
• ASA physical status I-III
• Male and female patients aged 18-65 years

Exclusion Criteria:

• Patients who did not provide consent
• Known allergy to propofol
• Body weight <40 kg
• Body mass index (BMI) >30 kg/m²
• Presence of acute or chronic pain
• Chronic alcohol consumption
• Use of antipsychotic, antidepressant, or hypnotic medications
• Patients with obstructive sleep apnea syndrome (STOP-Bang score ≥3)
• Patients with known anorectal diseases such as hemorrhoids, anal fissure, or anal fistula
• History of oncological disease
• Uncontrolled hypertension (blood pressure >180/110 mmHg)
• Severe hepatic or renal dysfunction
• Acute respiratory tract infection or other chronic respiratory disorders
• Patients with a history of colorectal surgery