Oral contraceptives and the prothrombin time

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Dr. De Teresa and others reported that mean prothrombin time ratio of 12 patients on long-term anticoagulation with warfarin was significantly higher when they were also taking oral contraceptives (OCs). A study of prothrombin complex activity was recently conducted in female rats treated with an estrogen-progestogen combination (lynestrenol 5 mg; mestranol 0.3 mg/kg body weight) which resulted in a 100% infertility in this species. After 1 treatment for only 1 estral cycle, OC-treated rats had a significantly longer Normotest clotting time (37.7+ or-0.5 sec) than control rats (31.0+or-0.4); the difference was even more notable after 10 cycles. Although this finding has not been reported in women on OCs, it may be that the estrogen-induced "lability" of the prothrombin complex occurs in humans only in special conditions, such as anticoagulation. Alternatively, liver dysfunction occurring among women on OCs may be responsible for reduced metabolism of warfarin, contributing to the effectiveness of the anticoagulation. Further pharmacology studies should be done to clarify the interaction between OCs and oral anticoagulants.

Original languageEnglish
Pages (from-to)332-333
Number of pages2
JournalBritish Medical Journal
Issue number6210
Publication statusPublished - 1980

ASJC Scopus subject areas

  • Medicine(all)


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