Narrow Therapeutic Index Drugs - Pharmacy Test Prep Study Guide for the NAPLEX, PTCB, NCLEX. Information that is useful for NAPLEX, PTCB, and NCLEX Exam Preparation. Therapeutic Index and Therapeutic Drug Monitoring definitions and explanations. Also includes specific NTI Medications and their monitoring parameters and signs and symptoms of toxicity including warfarin, digoxin, levothyroxine, cyclosporine, tacrolimus, carbamazepine, phenytoin, lithium and theophylline.