Background: Major depressive disorder (MDD), or depression, is a syndrome characterised by a number of behavioural, cognitive and emotional features. It is most commonly associated with a sad or depressed mood, a reduced capacity to feel pleasure, feelings of hopelessness, loss of energy, altered sleep patterns, weight fluctuations, difficulty in concentrating and suicidal ideation. There is a need for more effective and better tolerated antidepressants to combat this condition. Agomelatine was recently added to the list of available antidepressant drugs; it is a novel antidepressant that works on melatonergic (MT₁ and MT₂), 5-HT _2B and 5-HT_2C receptors. Because the mechanism of action is claimed to be novel, it may provide a useful, alternative pharmacological strategy to existing antidepressant drugs. Objectives: The objective of this review was 1) to determine the efficacy of agomelatine in alleviating acute symptoms of major depressive disorder in comparison with other antidepressants, 2) to review the acceptability of agomelatine in comparison with other antidepressant drugs, and, 3) to investigate the adverse effects of agomelatine, including the general prevalence of side effects in adults. Search methods: We searched the Cochrane Collaboration's Depression, Anxiety and Neurosis Review Group's Specialised Register (CCDANCTR) to 31 July 2013. The CCDANCTR includes relevant randomised controlled trials from the following bibliographic databases: CENTRAL (the Cochrane Central Register of Controlled Trials) (all years), EMBASE (1974 onwards), MEDLINE (1950 onwards) and PsycINFO (1967 onwards). We checked reference lists of relevant studies together with reviews and regulatory agency reports. No restrictions on date, language or publication status were applied to the search. Servier Laboratories (developers of agomelatine) and other experts in the field were contacted for supplemental data. Selection criteria: Randomised controlled trials allocating adult participants with major depression to agomelatine versus any other antidepressive agent. Data collection and analysis: Two review authors independently extracted data and a double-entry procedure was employed. Information extracted included study characteristics, participant characteristics, intervention details and outcome measures in terms of efficacy, acceptability and tolerability. Main results: A total of 13 studies (4495 participants) were included in this review. Agomelatine was compared to selective serotonin reuptake inhibitors (SSRIs), namely paroxetine, fluoxetine, sertraline, escitalopram, and to the serotonin-norepinephrine reuptake inhibitor (SNRI), venlafaxine. Participants were followed up for six to 12 weeks. Agomelatine did not show any advantage or disadvantage over the other antidepressants for our primary outcome, response to treatment (risk ratio (RR) 1.01, 95% confidence interval (CI) 0.95 to 1.08, P value 0.75 compared to SSRIs, and RR 1.06; 95% CI 0.98 to 1.16, P value 0.16 compared to venlafaxine). Also, agomelatine showed no advantage or disadvantage over other antidepressants for remission (RR 0.83; 95% CI 0.68 to 1.01, P value 0.07 compared to SSRIs, and RR 1.08; 95% CI 0.94 to 1.24, P value 0.73 compared to venlafaxine). Overall, agomelatine appeared to be better tolerated than venlafaxine in terms of lower rates of drop outs (RR 0.40; 95% CI 0.24 to 0.67, P value 0.0005), and showed the same level of tolerability as SSRIs (RR 0.95; 95% CI 0.83 to 1.09, P value 0.44). Agomelatine induced a lower rate of dizziness than venlafaxine (RR 0.19, 95% CI 0.06 to 0.64, P value 0.007). With regard to the quality of the body of evidence, there was a moderate risk of bias for all outcomes, due to the number of included unpublished studies. There was some heterogeneity, particularly between published and unpublished studies. The included studies were conducted in inpatient and outpatient settings, thus limiting the generalisability of the results to primary care settings. With regard to precision, the efficacy outcomes were precise, but the tolerability outcomes were mostly imprecise. Publication bias was variable and depended on the outcome of the trial. Our review included unpublished studies, and we think that this reduced the impact of publication bias. The overall methodological quality of the studies was not very good. Almost all of the studies were sponsored by the pharmaceutical company that manufactures agomelatine (Servier), and some of these were unpublished. Attempts to contact the pharmaceutical company Servier for additional information on all unpublished studies were unsuccessful. Authors' conclusions: Agomelatine did not seem to provide a significant advantage in efficacy over other antidepressive agents for the acute-phase treatment of major depression. Agomelatine was better tolerated than paroxetine and venlafaxine in terms of overall side effects, and fewer participants treated with agomelatine dropped out of the trials due to side effects compared to sertraline and venlafaxine, but data were limited because the number of included studies was small. We found evidence that compared agomelatine with only a small number of other active antidepressive agents, and there were only a few trials for each comparison, which limits the generalisability of the results. Moreover, the overall methodological quality of the studies was low, and, therefore, no firm conclusions can be drawn concerning the efficacy and tolerability of agomelatine.