Background:

Warfarin and selective serotonin reuptake inhibitors (SSRIs) are frequently co-prescribed in clinical practice, particularly among elderly patients with comorbid cardiovascular and psychiatric conditions. Although theoretical pharmacological mechanisms suggest an increased bleeding risk from their concurrent use, real-world evidence remains conflicting.

Objectives:

To comprehensively evaluate drug-drug interaction signals for bleeding events and international normalized ratio (INR) elevation associated with warfarin-SSRI combinations using large-scale pharmacovigilance databases from different healthcare systems (FDA Adverse Event Reporting System (FAERS) and Canada Vigilance Adverse Reaction Database (CVARD)).

Design:

A retrospective pharmacovigilance study.

Methods:

Adverse event reports from the FAERS (2004–2025) and the CVARD (1965–2025) were systematically analysed. Six warfarin-SSRI combinations were examined. Disproportionality and network analyses were utilized to characterize adverse event profiles. Multiple complementary signal detection algorithms were employed for interaction assessment, including the Omega shrinkage model, Additive Model, Multiplicative Model, and combination risk ratio (CRR) model, further supported by sensitivity analyses.

Results:

Disproportionality analysis revealed that the SSRIs&Warfarin group exhibited a significantly higher reporting odds ratio for international normalized ratio increased compared to the Warfarin-single group. However, regarding interaction signals, in the FAERS database, five of the six warfarin-SSRI combinations demonstrated consistent negative Ω values for bleeding events (range: −1.04 to −0.75), indicating no synergistic interaction. While the warfarin-fluvoxamine combination showed isolated positive signals in secondary models, the primary Ω shrinkage measure remained statistically nonsignificant (95% CI: encompassed zero). Results for international normalized ratio increased largely paralleled these findings. Network analysis confirmed a safety profile predominantly shaped by warfarin-associated risks rather than a synergistic amplification. The CVARD database corroborated these patterns with regional variations.

Conclusion:

Contrary to theoretical pharmacological expectations, this large-scale, cross-database pharmacovigilance analysis failed to detect positive safety signals for bleeding events or INR elevation associated with warfarin-SSRI combinations. Clinical vigilance and individualized risk assessment remain warranted, but routine combinations do not show synergistic bleeding risks at the population level.

Trial registration:

This study is a real-world pharmacovigilance investigation based on publicly accessible databases. As such, it does not involve a formal clinical trial, and therefore, no clinical trial registration number is applicable.

Plain language summary

Large-scale analysis finds no increased bleeding risk when blood thinner warfarin is combined with common antidepressants

Warfarin is a widely used blood thinner that prevents dangerous clots in patients with heart conditions. Selective serotonin reuptake inhibitors (SSRIs) are common antidepressants prescribed for depression and anxiety. Many patients, especially older adults, need both medications together. Based on how these drugs work in the body, doctors have worried that combining them might increase bleeding risk. This study analyzed over 900,000 reports from the US FDA adverse event database (2004–2025) and over 57,000 reports from the Canadian database (1990–2025). We used advanced statistical methods to detect whether combining warfarin with any of six SSRIs (escitalopram, citalopram, sertraline, paroxetine, fluvoxamine, or fluoxetine) increased reports of bleeding or elevated INR (a measure of blood thinning). Contrary to theoretical concerns, we found no consistent evidence that combining warfarin with SSRIs increases bleeding risk. Most combinations actually showed negative signals, meaning fewer bleeding reports than expected. When patients took both medications, their side effect profile closely resembled that of warfarin alone—not a combination of warfarin and SSRI effects. These findings were consistent across both US and Canadian databases. This suggests that real-world clinical practices—such as regular INR monitoring, careful dose adjustments, and heightened doctor awareness—may effectively manage any potential risks. However, this population-level finding does not mean individual patients face no risk. Doctors should still monitor patients starting or changing these medications, particularly those with additional risk factors like kidney problems or advanced age. This research provides reassuring evidence that necessary antidepressant treatment should not be withheld from patients taking warfarin due to bleeding concerns, while emphasizing that appropriate monitoring remains essential.