Such drugs have less than a twofold difference in the minimum toxic concentration and minimum effective concentration in the blood or are those drug product formulations that exhibit limited or erratic absorption, formulation-dependent bioavailability, and wide intrapatient pharmacokinetic variability that requires blood-level monitoring.” 2 As defined by the Therapeutic Products Directorate of Health Canada, an NTR drug is a drug wherein the ratio of the lowest concentration at which clinical toxicity occurs, to the median concentration providing a therapeutic effect, is less than or equal to two. The North Carolina Board of Pharmacy defines an NTI drug as “those pharmaceuticals having a narrowly defined range between risk and benefit.
