Oral contraceptives and venous thromboembolism: a quantitative discussion of the uncertainties.

OBJECTIVES: The majority of post-thrombotic women are barred from using oral contraceptives. We evaluated this policy for its clinical relevance.

DESIGN: A meta-analysis of controlled studies between 1960 and 1993.

SETTING: A Medline computer search, from 1966 to 1993, in multiple languages, with the following index terms: thrombosis, thrombopheblitis, vein, venous, pulmonary embolism, contraceptives, oestrogen, oral.

STUDY SELECTION: A total of 588 articles or abstracts were reviewed for controlled studies, in which an index group was compared with a control group. Included were one randomized trial, six follow-up studies and eight case-control studies.

MAIN OUTCOME MEASURES: Summary thrombosis risk for oral contraceptive users, number needed to discontinue oral contraceptives to prevent one (recurrent) thrombosis, comparison of additional unwanted pregnancies and postpartum thrombosis between alternative birth-control methods.

RESULTS: The studies proved highly heterogeneous with regard to size and direction of the risk estimate. The summary relative risk of first thrombosis during oral contraceptive use was 2.9 (95% CI, 0.5-17). Since the risk of thrombosis recurrence is not well known, we estimated alternatives, making various hypothetical assumptions, wherein women would continue to take oral contraceptives after a first episode of thrombosis, or stop and switch to use of an intra-uterine device, condom or the progestogen-only pill. Depending on the assumptions with regard to recurrence risk and the existence of possible subgroups with genetic coagulation defects, the cost-benefit ratio of advising against the use of oral contraceptives after a first thrombosis varied tremendously.

CONCLUSIONS: Our analysis shows that we lack the necessary data for recurrence risk of venous thrombosis during continuing use of oral contraceptives, or after switching to other modes of contraception. This reflects the clinical uncertainties that result in highly contradictory advice to young women who have experienced a first thrombosis. Only follow-up studies on recurrence risk will settle the issue.