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Many patients treated with vitamin K antagonists have unstable International Normalized Ratios (INRs), increasing the risk of thrombosis and bleeding events. Changes in dietary vitamin K intake are known to lead to INR variability. Therefore, it has been hypothesized that providing stable intake of vitamin K through daily supplementation may lead to decreased INR variability.Four studies – one retrospective and three prospective – have examined the effects of vitamin K supplementation on INR variability. The results of one retrospective study and two prospective studies of vitamin K1 supplementation (with doses ranging from 100 to 500 μg daily) in patients with a history of unstable INRs indicate that vitamin K supplementation may stabilize INRs, although the success varies among patients. Another prospective study of patients in a general anticoagulation clinic examined the effects of vitamin K1 100 μg daily on INR variability and found no difference in the time in the therapeutic range in patients randomized to vitamin K compared with placebo.Vitamin K supplementation may decrease variability of INRs in patients with a history of unstable INRs. Further studies are needed in larger populations to clarify the true effects of vitamin K supplementation.