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This article aims to determine whether chitosan, a popular, over-the-counter, weight loss supplement, is an effective treatment for overweight and obesity. It is designed as a systematic review of randomized controlled trials. The data sources include the electronic databases Medline, EMBASE, Biosis, CINAHL and Cochrane Central Register of Controlled Trials (CCTR); the specialized websites Controlled Trials, International Bibliographic Information on Dietary Supplements (IBIDS), System for Information on Grey Literature in Europe (SIGLE), Reuter's Health Service, Natural Alternatives International and Pharmanutrients; and bibliographies of relevant journal articles. Included were randomized controlled trials of chitosan with a minimum duration of 4 weeks in adults who were overweight or obese and/or had hypercholesterolaemia at baseline. Fourteen trials involving a total of 1071 participants were included in the review. Analyses involving all trials indicated that chitosan preparations result in a small but statistically significant greater reduction in body weight (weighted mean difference -1.7 kg; 95% confidence interval -2.1, -1.3 kg, P < 0.00001) compared with placebo. Analyses restricted to high-quality studies showed that reductions in weight [-0.6 (-1.2, 0.1) kg, P = 0.11] were less than in lower quality studies [-2.3 (-2.7, -1.8) kg, P < 0.00001]. Results obtained from high-quality trials indicate that the effect of chitosan on body weight is minimal and unlikely to be of clinical significance.