Complementary and Alternative Medicines Use in Patients on Warfarin and Matched Controls: A Retrospective Cohort Study

Abstract

ABSTRACTAimTo investigate complementary and alternative medicines (CAM) knowledge and use in patients on warfarin and matched controls, and the association between CAM use and adverse events (bleeding or clotting).MethodA retrospective cohort study was undertaken in the emergency department of a Melbourne hospital. Patients in the warfarin and control groups were those taking/not taking warfarin, respectively (groups matched for age and gender). An investigator‐administered questionnaire was used to collect data on CAM knowledge and use, and bleeding/clotting events during the previous 6 months. The international normalised ratio (INR) data during the previous 6 months were obtained from medical and pathology records.Results246 warfarin and 246 control group patients were enrolled. Approximately 50% in each group indicated that they knew of the potential for CAM–drug interactions. Only 60 (24%) patients in the warfarin group reported having been advised about CAM use. Compared to controls, a similar proportion of warfarin patients reported CAM use (29% vs 37%; p = 0.07) and significantly fewer used a CAM known to interact with warfarin (‘interacting CAM‘) (11% vs 19%; p = 0.03). In the warfarin group, CAM users and non‐users reported a similar number of events below and above their therapeutic INR range and when the INR exceeded 5. CAM users in the warfarin group tended to have more abnormal bleeding events (42% vs 30%; p = 0.06) but a similar number of clotting events (9% vs 10%; p = 0.8) than CAM non‐users. Warfarin patients who used ‘interacting CAM’ had significantly more clotting events than those who used CAM not known to interact with warfarin (18% vs 2%; p = 0.02).ConclusionCAM knowledge in people on warfarin was poor. ‘Interacting CAM' used concomitantly with warfarin may be associated with increased rates of clotting events.