The selective serotonin reuptake inhibitors (SSRIs) citalopram and escitalopram are associated with QT prolongation, which increases the risk of serious arrhythmia. Consequently, regulatory agencies issued safety warnings in 2011. This study aimed to investigate the risk of serious arrhythmia following initiation of citalopram or escitalopram compared to other SSRIs and the risk in the periods before and after the warnings were issued. We conducted a series of nationwide cohort studies emulating a target trial using Danish healthcare register data from January 1, 2002, to December 31, 2016. We included patients (aged ≥65 years) who filled an SSRI prescription with a 1-year washout period before the index date. The outcome was an event of serious arrhythmia. Individuals were followed for a maximum of 6 months using an intention-to-treat approach. Log-binomial regression analyses were performed, estimating risk ratios (RRs) and 95% confidence intervals (CIs) adjusting for age and sex, comorbidities, and comedications with propensity scores. Dose–response effects were not investigated because dosage instructions were not available. We included 167,366 (146,014 individuals), 40,113 (37,069 individuals), and 50,281 (44,754 individuals) person-trials of citalopram, escitalopram, and other SSRIs, respectively. In total, there were 228 events of serious arrhythmia. No difference in risk was observed in the entire study period for either citalopram (0.87 [0.62–1.22]) or escitalopram (0.85 [0.53–1.40]). We identified lower point estimates after the safety warning, RR 0.54 (95% CI 0.31–0.93) for citalopram and 0.58 (0.20–1.63) for escitalopram. Initiation of citalopram and escitalopram was not associated with an increased risk of serious arrhythmia. However, lower point estimates were observed after the safety warning.