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The efficacy of 5-aminosalicylic acids (5-ASAs) in ulcerative colitis (UC) has been studied previously in meta-analyses. However, several randomized controlled trials (RCTs) have been published recently, and no previous meta-analysis has studied the effect of 5-ASA dosage used.MEDLINE, EMBASE, and the Cochrane central register of controlled trials were searched (through December 2010). Eligible trials recruited adults with active or quiescent UC, comparing different doses of 5-ASAs with themselves or placebo. Dichotomous data were pooled to obtain relative risk (RR) of failure to achieve remission in active UC, and RR of relapse of disease activity in quiescent UC, with a 95% confidence interval (CI). The number needed to treat (NNT) was calculated from the reciprocal of the risk difference.The search identified 3,061 citations, and 37 RCTs were eligible. Of these, 11 compared 5-ASA with placebo in active UC remission, with the RR of no remission with 5-ASAs of 0.79 (95% CI 0.73–0.85; NNT=6). Doses of ≥2.0 g/day were more effective than <2.0 g/day for remission (RR=0.91; 95% CI 0.85–0.98). There were 11 RCTs comparing 5-ASAs with placebo in preventing relapse of quiescent UC, with the RR of relapse of 0.65 (95% CI 0.55–0.76; NNT=4). Doses of ≥2.0 g/day appeared more effective than <2.0 g/day for preventing relapse (RR=0.79; 95% CI 0.64–0.97).5-ASAs are highly effective for inducing remission and preventing relapse in UC. Evidence suggests that doses of ≥2.0 g/day have greater efficacy, although doses >2.5 g/day do not appear to lead to higher remission rates.