Abstract: We identified 3745 citations through electronic searches and reviews of reference lists after deduplication of search results. We excluded 3619 records during title and abstract review and assessed 126 full-text papers for inclusion in the review. Only one study, providing data of 225 participants, met our eligibility criteria and compared agomelatine (25 mg/day) with placebo. We rated it as having high risk of attrition bias because nearly half of the participants left the study before completion. We rated the certainty of the evidence as very low for all outcomes, because of high risk of bias, indirectness, and imprecision. The main analysis based on data of 199 participants rendered an indeterminate result with wide confidence intervals (CIs) that may encompass both a relevant reduction as well as a relevant increase of SAD incidence by agomelatine (risk ratio (RR) 0.83, 95% CI 0.51 to 1.34; 199 participants; very low-certainty evidence). Also the severity of SAD may be similar in both groups at the end of the study with a mean SIGH-SAD (Structured Interview Guide for the Hamilton Depression Rating Scale, Seasonal Affective Disorders) score of 8.3 (standard deviation (SD) 9.4) in the agomelatine group and 10.1 (SD 10.6) in the placebo group (mean difference (MD) -1.80, 95% CI -4.58 to 0.98; 199 participants; very low-certainty evidence). The incidence of adverse events and serious adverse events may be similar in both groups. In the agomelatine group, 64 out of 112 participants experienced at least one adverse event, while 61 out of 113 did in the placebo group (RR 1.06, 95% CI 0.84 to 1.34; 225 participants; very low-certainty evidence). Three out of 112 patients experienced serious adverse events in the agomelatine group, compared to 4 out of 113 in the placebo group (RR 0.76, 95% CI 0.17 to 3.30; 225 participants; very low-certainty evidence). No data on quality of life or interpersonal functioning were reported. We did not identify any studies on melatonin. Given the uncertain evidence on agomelatine and the absence of studies on melatonin, no conclusion about efficacy and safety of agomelatine and melatonin for prevention of SAD can currently be drawn. The decision for or against initiating preventive treatment of SAD and the treatment selected should consider patient preferences and reflect on the evidence base of all available treatment options.

Plain-language summary: The included study showed neither a clear effect in favour nor against agomelatine as a preventive treatment. In addition, the certainty of evidence for all outcomes was very low, making it impossible to draw any conclusions about the efficacy and safety of agomelatine for the prevention of winter depression. No evidence on melatonin for prevention of SAD was identified. Doctors need to discuss with persons with a history of SAD that currently evidence on agomelatine or melatonin for preventive treatment options for SAD is inconclusive, therefore treatment selection should be strongly based on peoples' preferences and reflect on the evidence base of all available treatment options. Review authors recommend that future studies should evaluate the efficacy of agomelatine or melatonin in preventing SAD and should directly compare these interventions against other treatment options, such as light therapy, antidepressants, or psychological therapies to determine the best treatment option for prevention of SAD.