Limited knowledge on progestogen-only contraception and risk of venous thromboembolism

Objective. To assess the current knowledge concerning progestogen-only contraception (POC) and risks of venous thromboembolism (VTE). Design and setting. Systematic review of the literature on observational and analytical studies reporting risk estimates for VTE in women exposed to POCs. Methods and main outcome measures. We performed a computerized literature search in the Pub Med, Embase, and the Cochrane Library for studies published between 1966 and February 13, 2008. Based on the evaluated studies we calculated an overall risk estimate for VTE in association with POC. Results. Four case-control studies and one cohort study were included. Of the case-control studies, three reported an increased risk and one a decreased risk of VTE. The cohort study found divergent results depending on the type of statistical analysis used. None of the results was statistically significant. The overall odds ratio for POC-associated VTE in the four case-control studies was 1.45 (95% CI=0.92-2.26). Conclusions. The risk of VTE associated with use of POCs is poorly investigated. The slightly elevated overall risk estimate might suggest an association between POC and an increased risk for VTE. The results must, however, be interpreted with caution due to the possibility of residual confounding. Well-designed studies with sufficient statistical power to evaluate risks of VTE with POC are warranted.