Endoscopic Retrograde Cholangiopancreatography (ERCP) is a widely used procedure for diagnosing and treating pancreatic and biliary diseases. Despite its benefits, ERCP carries a risk of post-procedure pancreatitis (PEP), which occurs in approximately 12.2% of cases and can significantly increase healthcare costs and patient morbidity. Preventing PEP is crucial for improving patient outcomes and reducing the economic burden of ERCP. Nonsteroidal anti-inflammatory drugs (NSAIDs), such as indomethacin, have been shown to be effective in reducing the incidence of PEP when administered rectally before ERCP. The standard dose recommended by guidelines is 100mg, which has been associated with a significant reduction in PEP rates. However, higher doses of NSAIDs can increase the risk of adverse events, including gastrointestinal bleeding and renal impairment. Therefore, there is a need to determine whether a lower dose can provide similar benefits without increasing these risks. This multicenter, non-inferiority, double-blind, randomized controlled trial will be conducted in China. Participants will be adults aged 18 or older scheduled for ERCP. They will be randomly assigned in a 1:1 ratio to either the low-dose (50mg) or standard-dose (100mg) indomethacin group. The intervention will be administered rectally 30 minutes before the ERCP procedure. The study will follow a double-blind design, ensuring that both patients and investigators are unaware of the treatment allocation. The results of this trial could significantly influence clinical practice by providing evidence on the effectiveness of a lower dose of indomethacin in preventing PEP. This could lead to a reduction in the risk of adverse events associated with higher doses and potentially decrease healthcare costs without compromising patient safety. By optimizing the dosing of indomethacin, this study aims to improve the safety and cost-effectiveness of ERCP procedures.