ObjectiveThis meta-analysis aimed to comprehensively explore the risk factors for inadequate bowel preparation (IBP).MethodsWe searched the Embase, PubMed, Web of Science, and The Cochrane Library databases up to August 24, 2023, to identify observational studies and randomized controlled trials (RCTs) that examined risk factors for IBP. A random effects model was used to pool the adjusted odds ratios and 95% confidence intervals.ResultsA total of 125 studies (91 observational studies, 34 RCTs) were included. Meta-analyses of observational studies revealed that three preparation-related factors, namely, characteristics of last stool (solid or brown liquid), incomplete preparation intake, and incorrect diet restriction, were strong predictors of IBP. The other factors were moderately correlated with IBP incidence, including demographic variables (age, body mass index, male sex, Medicaid insurance, and current smoking), comorbidities (diabetes, liver cirrhosis, psychiatric disease, Parkinson's disease, previous IBP, poor mobility, inpatient, and Bristol stool form 1/2), medications (tricyclic antidepressants, opioids, antidepressants, narcotics, antipsychotics, and calcium channel blockers), and preparation-related factors (preparation-to-colonoscopy interval not within 3 to 5/6 h, nonsplit preparation, and preparation instructions not followed). No colonoscopy indications were found to be related to IBP. Meta-analyses of RCTs showed that education, constipation, stroke/dementia, and discomfort during preparation were also moderately associated with IBP. Most of the other findings were consistent with the pooled results of observational studies. However, primarily due to imprecision and inconsistency, the certainty of evidence for most factors was very low to moderate.ConclusionsWe summarized five categories of risk factors for IBP. Compared to demographic variables, comorbidities, medications, and colonoscopy indications, preparation-related elements were more strongly associated with IBP. These findings may help clinicians identify high-risk individuals and provide guidance for IBP prevention.