BACKGROUND: Premenstrual syndrome (PMS) is a combination of physical, psychological and social symptoms in women of reproductive age, and premenstrual dysphoric disorder (PMDD) is a severe type of the syndrome, previously known as late luteal phase dysphoric disorder (LLPDD). Both syndromes cause symptoms during the two weeks leading up to menstruation (the luteal phase). Selective serotonin reuptake inhibitors (SSRIs) are increasingly used as a treatment for PMS and PMDD, either administered in the luteal phase or continuously. We undertook a systematic review to assess the evidence of the positive effects and the harms of SSRIs in the management of PMS and PMDD. OBJECTIVES: To evaluate the benefits and harms of SSRIs in treating women diagnosed with PMS and PMDD. SEARCH METHODS: We searched the Cochrane Gynaecology and Fertility (CGF) Specialised Register of Controlled Trials, CENTRAL, MEDLINE, Embase and PsycINFO for randomised controlled trials (RCTs) in November 2023. We checked reference lists of relevant studies, searched trial registers and contacted experts in the field for any additional trials. This is an update of a review last published in 2013. SELECTION CRITERIA: We considered studies in which women with a prospective diagnosis of PMS, PMDD or LLPDD were randomised to receive SSRIs or placebo. DATA COLLECTION AND ANALYSIS: We used standard Cochrane methods. We pooled data using a random-effects model. We calculated standardised mean differences (SMDs) with 95% confidence intervals (CIs) for premenstrual symptom scores, using 'post-treatment' scores for continuous data. We calculated odds ratios (ORs) with 95% CIs for dichotomous outcomes. We stratified analyses by type of administration (luteal phase or continuous). We calculated absolute risks and the number of women who would need to be taking SSRIs in order to cause one additional adverse event (i.e. the number needed to treat for an additional harmful outcome (NNTH)). We rated the overall certainty of the evidence for the main findings using GRADE. MAIN RESULTS: We included 34 RCTs in the review. The studies compared SSRIs (i.e. fluoxetine, paroxetine, sertraline, escitalopram and citalopram) to placebo. SSRIs probably reduce overall self-rated premenstrual symptoms in women with PMS and PMDD (SMD -0.57, 95% CI -0.72 to -0.42; I = 51%; 12 studies, 1742 participants; moderate-certainty evidence). SSRI treatment was probably more effective when administered continuously than when administered only in the luteal phase (P = 0.03 for subgroup difference; luteal phase group: SMD -0.39, 95% CI -0.58 to -0.21; 6 studies, 687 participants; moderate-certainty evidence; continuous group: SMD -0.69, 95% CI -0.88 to -0.51; 7 studies, 1055 participants; moderate-certainty evidence). The adverse effects associated with SSRIs were nausea (OR 3.30, 95% CI 2.58 to 4.21; I = 0%; 18 studies, 3664 women), insomnia (OR 1.99, 95% CI 1.51 to 2.63; I = 0%; 18 studies, 3722 women), sexual dysfunction or decreased libido (OR 2.32, 95% CI 1.57 to 3.42; I = 0%; 14 studies, 2781 women), fatigue or sedation (OR 1.52, 95% CI 1.05 to 2.20; I = 0%; 10 studies, 1230 women), dizziness or vertigo (OR 1.96, 95% CI 1.36 to 2.83; I = 0%; 13 studies, 2633 women), tremor (OR 5.38, 95% CI 2.20 to 13.16; I = 0%; 4 studies, 1352 women), somnolence and decreased concentration (OR 3.26, 95% CI 2.01 to 5.30; I = 0%; 8 studies, 2050 women), sweating (OR 2.17, 95% CI 1.36 to 3.47; I = 0%; 10 studies, 2304 women), dry mouth (OR 2.70, 95% CI 1.75 to 4.17; I = 0%; 11 studies, 1753 women), asthenia or decreased energy (OR 3.28, 95% CI 2.16 to 4.98; I = 0%; 7 studies, 1704 women), diarrhoea (OR 2.06, 95% CI 1.37 to 3.08; I = 0%; 12 studies, 2681 women), and constipation (OR 2.39, 95% CI 1.09 to 5.26; I = 0%; 7 studies, 1022 women). There was moderate-certainty evidence for all adverse effects other than somnolence/decreased concentration, which was low-certainty evidence. Overall, the certainty of the evidence was moderate. The main weakness was poor reporting of study methodology. Heterogeneity was low or absent for most outcomes, although there was moderate heterogeneity in the analysis of overall self-rated premenstrual symptoms. Based on the meta-analysis of response rate (the outcome with the most included studies), there was suspected publication bias. In total, 68% of the included studies were funded by pharmaceutical companies. This stresses the importance of interpreting the review findings with caution. AUTHORS' CONCLUSIONS: SSRIs probably reduce premenstrual symptoms in women with PMS and PMDD and are probably more effective when taken continuously compared to luteal phase administration. SSRI treatment probably increases the risk of adverse events, with the most common being nausea, asthenia and somnolence.