Therapeutic Index
The ratio between the toxic dose and therapeutic dose of a drug. A wide therapeutic index means a large safety margin. Most peptide drugs have relatively wide therapeutic indices due to their receptor specificity, though the index may differ depending on the indication being treated.
Technical Context
TI = TD50/ED50 (ratio of dose causing toxicity in 50% of population to dose producing desired effect in 50%). A TI >10 is generally considered wide; TI <2 is narrow. Drugs with narrow TI include: vancomycin (TI ~2-4, requiring therapeutic drug monitoring), warfarin (TI ~2), lithium (TI ~2-3), and digoxin (TI ~2). Most peptide drugs have wider TI due to receptor specificity, but TI varies by indication — semaglutide's TI for glycaemic control may differ from its TI for weight management (where higher doses are used). The therapeutic index concept is also applied to the separation between desired effects and specific adverse effects: for GLP-1 RAs, the 'therapeutic index' for weight loss vs nausea is relatively narrow during dose titration.