Perhaps the biggest challenge with warfarin is the relationship between increasing serum levels of the drug, as measured by the international normalized ratio (INR), and the increasing risk for intracranial hemorrhage (ICH; Figure 8). This is a reflection of warfarin’s very narrow therapeutic index.[7] If the INR drops below approximately 2, the risk of ischemic stroke goes up markedly. Likewise, as the INR goes above 3 and in particular if it goes above 4, the risk of ICH goes up rather steeply.

This narrow therapeutic index means that frequent INR monitoring is necessary, because any of the many factors that can affect the dose-to-INR relation (such as drug–drug or drug–food interactions) can disrupt whether the patient’s warfarin is within the therapeutic range and thereby seriously affect the odds for risk of stroke or, conversely, ICH.  Bhatt DL. Am J Med 2014; 00.