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

Abstract

BackgroundThe incidence rate of venous thrombosis (VT) in women switching combined oral contraceptives (COCs) is unknown. ObjectivesWe 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. MethodsWe 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. ResultsIn 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). ConclusionSwitchers, particularly in the first 3 months after switching, may experience a renewed starter effect thereby increasing the risk of VT.