It is unclear how polyethylene glycol (PEG) laxatives compare with other classes of laxative in terms of efficacy. To assess efficacy of PEG vs. placebo and active comparators in adults with non-organic constipation. Text Word searches were carried out on MEDLINE, EMBASE, CINAHL, Cochrane Central Register of Clinical Trials and Google Scholar databases covering the period January 1970 to October 2009. Search terms were (constipation) AND (randomised OR randomized) AND (PEG OR polyethylene OR macrogol OR movicol OR idrolax OR miralax OR transipeg OR forlax OR colyte OR golytely OR isocolan OR nulytely) NOT colonoscopy. Only published randomised controlled trials, with a parallel-group or cross-over design, comparing oral PEG with placebo or a comparator laxative in adults with a history of non-organic constipation, were included. The frequency of defaecation in each arm, on completion of the protocol-defined treatment duration was extracted. All pooled analyses were based on random effect models. Of the 20 qualifying studies, 10 were vs. placebo, seven were vs. lactulose, and four were vs. other agents. One study compared PEG, placebo and lactulose. PEG treatment resulted in a highly significant increase in defaecations/week over placebo (all studies: additional 1.98 stools/week; p = 0.0003, high-quality studies: additional 2.34 stools/week; p = 0.0001) and over lactulose (all studies: additional 1 stool/week; p = 0.0017, high-quality studies: additional 1.65 stools/week; p = 0.021). This meta analysis is the only quantitative statistical analysis to have been published in the field. PEG was found to be a more effective laxative than lactulose in adult patients with constipation.