Incidence rate of venous thrombosis in women switching combined oral contraceptives: a cohort study

Deeksha Khialani; Esther de Rooij; Szimonetta Komjáthiné Szépligeti; Elena Dudukina; Saskia le Cessie; Vera Ehrenstein; Frits R Rosendaal; Astrid van Hylckama Vlieg

doi:10.1016/j.rpth.2024.102390

Incidence rate of venous thrombosis in women switching combined oral contraceptives: a cohort study

Res Pract Thromb Haemost. 2024 Mar 27;8(3):102390. doi: 10.1016/j.rpth.2024.102390. eCollection 2024 Mar.

Authors

Deeksha Khialani¹, Esther de Rooij¹, Szimonetta Komjáthiné Szépligeti², Elena Dudukina², Saskia le Cessie^{1

3}, Vera Ehrenstein², Frits R Rosendaal^{1

4}, Astrid van Hylckama Vlieg¹

Affiliations

¹ Department of Clinical Epidemiology, Leiden University Medical Center, Leiden, The Netherlands.
² Department of Clinical Medicine - Department of Clinical Epidemiology, Aarhus University, Aarhus University Hospital, Aarhus, Denmark.
³ Department of Biomedical Data Sciences, Leiden University Medical Center, Leiden, The Netherlands.
⁴ Department of Thrombosis & Hemostasis, Leiden University Medical Center, Leiden, The Netherlands.

Abstract

Background: The incidence rate of venous thrombosis (VT) in women switching combined oral contraceptives (COCs) is unknown.

Objectives: We hypothesize that women switching COCs may have a similar increased incidence rate of VT as women who start COCs. Switching means starting with a new COC, which may biologically approximate starting.

Methods: We conducted a cohort study with data from the Netherlands and Denmark. First, we identified starters who were defined as women who did not use COCs in the 2 years prior to the start of their first COC prescription within the study period. Switchers were a subset of COC starters who redeemed a COC formulation different from their initial COC during follow-up but not longer than 12 months after starting. We estimated incidence rate ratios (adjusted incidence rate ratio [aIRR]) of VT with 95% CIs among COC switchers as compared with COC starters using Poisson regression adjusted for age, COC progestogen generation, and preexisting obesity.

Results: In both countries, we found an increased risk of VT among switchers as compared with starters during the first 3 months of the follow-up (aIRR = 1.77; 95% CI, 1.22-2.56 in the Netherlands and aIRR = 1.50; 95% CI, 1.04-2.16 in Denmark).

Conclusion: Switchers, particularly in the first 3 months after switching, may experience a renewed starter effect thereby increasing the risk of VT.

Keywords: cohort studies; combined oral contraceptives; incidence rate; venous thrombosis; women.