BACKGROUND: Mirtazapine is used to treat depression worldwide, and the effects of mirtazapine on depression rating scales are well-known. Our primary objective was to assess the risks of adverse events with mirtazapine for major depressive disorder. METHODS: We searched relevant sources from inception to 7 March 2024 for randomised clinical trials comparing mirtazapine versus placebo in adults with major depressive disorder. The primary outcomes were suicides or suicide attempts, serious adverse events, and non-serious adverse events. Data were synthesised using meta-analysis and Trial Sequential Analysis. RESULTS: We included 17 trials randomising 2,131 participants to mirtazapine versus placebo. All results were at high risk of bias, and the certainty of the evidence was very low. The included trials assessed outcomes at a maximum of 12 weeks after randomisation. Meta-analysis and Trial Sequential Analysis showed insufficient information to determine the effects of mirtazapine on the risks of suicides or suicide attempts and serious adverse events. Meta-analyses showed that mirtazapine increased the risks of somnolence, weight gain, dry mouth, dizziness, and increased appetite but decreased the risk of headaches. CONCLUSIONS: There is a lack of evidence on the effects of mirtazapine on suicides and serious adverse events. Mirtazapine increases the risks of somnolence, weight gain, dry mouth, dizziness, and increased appetite. Mirtazapine might decrease the risk of headaches. The long-term effects of mirtazapine are unknown. PROSPERO ID: CRD42022315395.