Colonoscopy requires bowel cleansing for gut mucosa visualization; high-quality cleansing facilitates lesion detection. NER1006 is a 1L polyethylene glycol (PEG) bowel preparation. This post hoc analysis of two randomized trials investigated cleansing efficacy assessed, as in clinical practice, by site endoscopists. Patients received NER1006, 2L PEG + ascorbate (2LPEG), or oral sulfate solution (OSS) as a 2-day evening/morning regimen (N2D) or NER1006 morning-only dosing (N1D). Treatment-blinded site endoscopists assessed cleansing using the Harefield Cleansing Scale (HCS). Analyses were conducted in a modified full analysis set, including (mFAS; n = 1378) or excluding (mFAS2; n = 1319) imputed failures, and in patients with 100% treatment adherence (mFAS100; n = 1047). Overall cleansing success (HCS grade A/B), overall high-quality cleansing (HCS grade A), and high-quality segments (HCS 3–4) per treatment population were analyzed. Overall cleansing success was higher with N2D than 2LPEG (92.7–97.5% vs. 87.9–93.0%), and more patients had overall high-quality cleansing with N2D and N1D than 2LPEG (68.0–72.1% and 64.0–68.4% vs. 50.7–56.0%). Without imputed failures, N2D delivered more overall high-quality cleansing than OSS (74.5–77.3% vs. 67.8–69.8%). More high-quality segments were demonstrated with N2D and N1D versus 2 LPEG (82.5–87.1% and 79.4–84.4% vs. 70.4–76.3%) and with N2D versus OSS (82.7–89.5% vs. 78.1–84.4%). When assessed by site endoscopists, NER1006 delivers greater high-quality cleansing than 2LPEG or OSS.