Fifty-four trials (n=9,268 patients) were included in the review. Follow-up duration of post-randomisation treatment in the trials was classified as either short term (24 weeks, range seven to 24 weeks) and long term (range 24 to 156 weeks).
All antidepressant medications were found to significantly reduce the risk of relapse during continuation treatment compared with placebo (OR 0.38, 95% CI 0.34 to 0.41). Statistically significant heterogeneity was also observed (I2=79%, χ2=19.27) across the trials, which was found to be due to the results of monoamine oxidase inhibitors. When these data were removed, heterogeneity was not statistically significant.
Similar patterns of results showing significant benefits of continued antidepressant treatment were also observed for elderly patients (OR 0.30, 95% CI 0.22 to 0.41; eight trials; n=775 patients) and non-elderly patients (OR 0.39, 95% CI 0.35 to 0.42; 46 trials; n=8,503 patients), DSM III-IV major depressive disorder (OR 0.39, 954% CI 0.35 to 0.43; 31 trials; n=7,239 patients), major depressive disorder classified using non-DSM methods (OR 0.24, 95% CI 0.17 to 0.32; 14 trials; n=731 patients), and patients in mixed depressive states (OR 0.43 95% CI 0.33 to 0.55; n=1,298 patients). The results were similar when analysed using different statistical methods, although random-effects odds ratios were slightly lower than those calculated using fixed-effects models. Earlier trials also had lower odds ratios (Spearman's r=0.331, p=0.01).
Significant benefits of continuation treatment with antidepressants were also observed at short-term, intermediate and long-term follow-up.
Statistical heterogeneity was also substantial across the results for the analyses by age (I2=52%, χ2=2.08), diagnostic criteria (I2=79%, χ2=9.70) and treatment duration (I2=62%, χ2 =10.43).