Prevalence of Use and the Risk of Adverse Effects Associated with Complementary and Alternative Medicine in a Cohort of Patients Receiving Warfarin

Abstract

The use of complementary and alternative medicine (CAM), including orally administered herbals, botanicals, vitamins, and supplements, may pose a risk to patients on warfarin therapy. To estimate the prevalence of CAM use among patients taking warfarin and evaluate the impact of CAM exposure on the risk of warfarin-related adverse effects. A survey was administered to hospital inpatients and clinic outpatients on drug exposure (including CAM) over the previous month, self-reported bleeding events, use of alcohol and vitamin K-rich foods, and medical conditions. Prescription medication use was verified, and laboratory records were checked for out-of-range international normalized ratios (INRs) (defined as INR >4 or <2). The use of CAM, including products with reported or theoretical interactions with warfarin, was compared between patients with and without self-reported bleeding or out-of-range INR. Among the 314 patients who completed the survey, 44.3% reported using CAM at least weekly. Potentially interacting CAM was used by 34.1% of all patients, or 18.2% if vitamin E was excluded as an interacting CAM. Vitamin E was used by 24.2% of all patients and 71.0% of those who used potentially interacting CAM. There was no significant difference in CAM use or consumption of vitamin K-rich foods between patients with and without INRs greater than 4 or for patients with and without INRs less than 2. The use of potentially interacting CAM in this cohort was higher than the use previously reported among patients on warfarin therapy. However, exposure to CAM was not associated with an increase in the risk of self-reported bleeding or out-of-range INR.