Time of day for taking warfarin does not affect the stability of the drug’s anticoagulant effect, according to a new study.
Warfarin is commonly recommended to be taken in the evening. However, there has been no evidence to support whether the administration time has a clinically significant effect on the drug’s efficacy, the study authors wrote.
The study, published in the Annals of Family Medicine, evaluated the effect of taking warfarin in the evening versus morning across the offices of 236 primary care clinicians. Warfarin users were randomized to take the medication in the evening either to switch to morning administration or to continue evening use. Time in the therapeutic range (TTR) was assessed to detect differences in the stability of warfarin’s anticoagulant effect.
Of the 217 participants, 109 switched to morning warfarin and 108 continued evening use. According to the results, TTR increased from 71.8% to 74.7% in the morning group, and from 72.6% to 75.6% in the evening group, for a change in TTR of 2.9% in the former versus 3.0% in the latter (difference, -0.1%; P=.97; 95% CI for the difference, -6.1% to 5.9%).
The difference in percent change in proportion of time outside the therapeutic INR range (obtained via Hodges-Lehmann estimation of the difference in medians) was 4.4% (P=.62; 95% CI for the difference, - 17.6% to 27.3%), the data showed.
Overall, the researchers concluded that warfarin administration time, morning versus evening, had no clinically important effect on the proportion of time that warfarin users spend outside the therapeutic range. This held true regardless of self-reported frequency and variability with which foods containing high amounts of vitamin K were consumed, they noted.
“Given that patients and their community caregivers might find a particular time of day more convenient or easier to remember, warfarin administration time should be tailored to patient preference,” the researchers concluded.
- Garrison SR, Green L, Kolber MR, et al. The effect of warfarin administration time on anticoagulation stability (INRange): A pragmatic randomized controlled trial. Annals of Family Medicine. 2020. Doi: 10.1370/afm.2488