The combination of different laxatives at reduced volumes may benefit patients by enhancing efficacy for bowel cleansing and increasing tolerability. However, evidence regarding combined preparations is scarce. This study evaluated whether the combined preparations are associated with enhanced efficacy and tolerability. This randomized phase II study had a single-blind, parallel-arm design. Between December 2013 and September 2014, consecutive patients aged between 20 and 65 years and who required diagnostic colonoscopies were considered for inclusion. Patients were randomly allocated into 4 arms: sodium picosulfate and magnesium citrate (PMC) and polyethylene glycol (PEG) with ascorbic acid in a day-prior (PMC-PEG-DP), PMC and oral sodium phosphate (NaP) in a day-prior (PMC-NaP-DP), PMC and PEG with ascorbic acid in a split-dose (PMC-PEG-SD), and PMC and oral NaP in a split-dose (PMC-NaP-SD). Primary endpoint was the Aronchick scale, and Ottawa scale results by colon segment, patients' adverse gastrointestinal symptoms, and willingness to reuse the same agents were also recorded. Successful bowel preparation was defined as an "excellent" or "good" score on the Aronchick scale. A total of 236 patients were randomized and 229 patients received the planned colonoscopy. The rates of successful bowel preparation in the PMC-PEG-DP, PMC-NaP-DP, PMC-PEG-SD, and PMC-NaP-SD were 82.5%, 64.4%, 100%, and 100%, respectively. Excluding the failed PMC-NaP-DP group, all groups showed satisfactory rates of successful bowel preparation, and the mean Ottawa scores were significantly better in the PMC-PEG-SD and PMC-NaP-SD groups than in the PMC-PEG-DP group (P < 0.0001). The PMC-PEG-DP, PMC-NaP-DP, PMC-PEG-SD, and PMC-NaP-SD groups were similar in terms of rates of adverse gastrointestinal symptoms reported on a 5-point scale (P = 0.40) and willingness to reuse the same combined preparations (P = 0.55). PMC-PEG in a day-prior or split-dose and PMC-NaP in a split-dose were efficient and tolerable bowel preparations for colonoscopy.