Background <p>Lactulose is a novel laxative with low volume and excellent tolerability. It used as a bowel preparation for colonoscopy has so far limited application. We compared the effectiveness, tolerability, and safety of lactulose with those of polyethylene glycol (PEG) in bowel preparation of patients with inflammatory bowel disease (IBD).</p> Methods <p>We enrolled 140 patients with clinically inactive IBD who required colonoscopy and randomly divided them into the 3L-PEG group or the 1.5L-Lactulose group. Bowel cleansing was assessed using the Boston Bowel Preparation Scale. The tolerability and satisfaction of the bowel preparation methods were evaluated using self-administered questionnaires. Safety was assessed by monitoring electrolytes, and liver and kidney functions in partial patients.</p> Results <p>The successful bowel preparation rate was higher in the Lactulose group than in the PEG group (92.9% versus 81.4%, <i>P</i> = 0.0434). According to the multivariable analyses, the male sex was an independent risk factor of efficacy. The responses to self-administered questionnaires revealed that the Lactulose group experienced significantly better taste and had a higher willingness to repeat the same regimen than the PEG group (<i>P</i> &lt; 0.0001). In the Lactulose group, most patients who had used PEG before could better evaluate lactulose (<i>P</i> &lt; 0.0001). No significant differences in adverse events and safety were between the two groups (<i>P</i> &gt; 0.05). The multivariable analyses revealed age was an independent factor of adverse events.</p> Conclusions <p>1.5L-Lactulose had a higher bowel cleansing success rate and higher tolerability with acceptable safety than 3L-PEG. Therefore, 1.5L-Lactulose may be a potential option for bowel preparation in inactive IBD patients.</p>