Oral contraceptives and fatal thromboembolism in young women.

Abstract

Authors of a research article in an earlier edition of the International Journal of Epidemiology have not found an increased risk of fatal venous thromboembolism in young women who used oral contraceptives (OCs). They think that the odds ratio of fatal venous thromboembolism associated with current use of OCs is not as great as it was in the past thereby explaining the lack of association. The trend towards lower dosages of estrogen and progestogens in OCs may account for this trend. Even though some studies have demonstrated a gradation in risk of thromboembolism linked to estrogen dose others do not show a diminished risk. The authors of the earlier article have failed to note that selective prescription of OCs may have affected their results a significant bias. For example since physicians have known for 25 years of the link between OCs and venous thromboembolism they have likely prescribed alternative dose OCs to women at high risk of thromboembolism (i.e. smokers women who recently experienced an operation or women with a history of venous thrombosis). In fact in the study in question just 1 of the 12 controls excluded for previous morbidity was a current OC user suggesting selective prescription of OCs which was much less than among the controls used in the study. A population at risk which has a lower prevalence of OC use would mask any possible link between use of low dose OCs and fatal thromboembolism. In the Netherlands epidemiologists have not found a decrease in fatal thromboembolism in young women even though they did expect a decrease. Thus there are probably other factors besides OCs causing fatal thromboembolism in young women.