What is a narrow therapeutic index and which drugs have one?

A narrow therapeutic index means the margin between the effective dose and the toxic dose is very small, requiring careful dose adjustment and frequent monitoring. Drugs with narrow therapeutic indexes include warfarin monitored by INR, lithium monitored by serum levels, digoxin monitored by serum levels, theophylline, phenytoin, aminoglycosides like gentamicin monitored by peak and trough levels, and vancomycin monitored by trough levels. Small changes in dose, absorption, metabolism, or drug interactions can push levels from therapeutic to toxic.