FIRST-TIME USE OF NEWER ORAL-CONTRACEPTIVES AND THE RISK OF VENOUS THROMBOEMBOLISM

Recent epidemiologic studies reported that the risk of venous thromboe mbolism (VTE) was higher with the use of the newer third generation or al contraceptives than with second generation agents. Although the ove rall findings of these studies are similar, the results, as they relat e to patterns and duration of oral contraceptive use particularly amon g first-time users, are inconsistent. We reanalyzed data from the Tran snational case-control study to assess the risk of VTE associated with first-time use of oral contraceptives as a function of its duration o f use. Over the period 1993 to 1995, 471 cases of venous thromboemboli sm were identified in Germany and the United Kingdom. For each case, u p to four controls were obtained, for a total of 1772 controls. Data o n oral contraceptive use and confounding variables, including data on sociodemographic, lifestyle, medical history, and family history of di sease, were obtained by interview. Data analysis was based on the 105 cases and 422 controls who were first-time users of second or third ge neration agents, or never users of oral contraception. Rate ratios, ad justed for confounders and approximated by odds ratios, were estimated as a continuous function of duration of oral contraceptive use by log istic regression and quadratic spline models. We found, for first-time users, that the adjusted rate ratio of VTE as a function of the durat ion of oral contraceptive use is essentially identical for second and third generation pills relative to never users. This rate ratio increa ses to around 10 in the first year of use and decreases to around two after 2 years of use, remaining at this risk level thereafter for both second and third generation agents. We conclude that second and third generation agents are associated with identical risks of venous throm boembolism when they are prescribed to women who are using oral contra ceptives for the first time ever. (C) 1997 Elsevier Science Inc. All r ights reserved.