(HealthDay News) — Time of day for taking warfarin makes no difference on the drug’s effectiveness, according to a study published in the January/February issue of the Annals of Family Medicine.

Scott R. Garrison, MD, PhD, from University of Alberta in Edmonton, Canada, and colleagues evaluated the effect of administration time on the stability of warfarin’s anticoagulant effect among community-dwelling warfarin users, with 109 randomly assigned to morning use and 108 to evening use. The proportion of time within the therapeutic range (TTR) of the international normalized ratio (INR) was compared between blood test month 2 to month 7 postrandomization versus 6 months prerandomization.

The researchers found that TTR rose from 71.8 to 74.7% in the morning group and from 72.6 to 75.6% in the evening group (difference, −0.1%; 95% confidence interval, −6.1 to 5.9%; P=0.97). Using the Hodges-Lehmann estimation of the difference in medians, the difference in percent change in proportion of time outside the therapeutic INR range was 4.4% (95% confidence interval for the difference, −17.6 to 27.3%; P=0.62).

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“Patients should take warfarin whenever regular compliance would be easiest,” the authors write.


Garrison SR, Green L, Kolber MR, et al. The Effect of Warfarin Administration Time on Anticoagulation Stability (INRange): A Pragmatic Randomized Controlled Trial. Ann Fam Med.

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