Overview Therapeutic index (TI) measurement of drug safety refers to the relationship between toxic and therapeutic dosing TI = TD50/ED50 therapeutic index of a drug is the ratio of the dose that produces toxicity to the dose that produces a clinically desired or effective response TD50 = the dose of drug that causes a toxic response in 50% of the population ED50 = the dose of drug that is therapeutically effective in 50% of the population a safer drug has a higher therapeutic index very large lethal dose, very small effective dose a more dangerous drug has a lower therapeutic index may require regular monitoring of drug levels examples include: warfarin lithium digoxin phenytoin gentamicin amphotericin 5-fluoro-urocil zidovudine Therapeutic drug monitoring therapeutic window (TW) is the range of drug concentrations in which a drug is effective TW concept is reflected in TI calculation TW = MTC/MEC Minimum effective concentration (MEC) is the minimum concentration that is required for drug effect Minimum toxic concentration (MTC) is the minimum concentration in which toxicity usually occurs If you have a low dose for the drug to start working (MEC) and a high dose at which the drug is toxic (MTC) you have a wide window in which the drug can safely be used (TW)