Question 1

What makes a drug narrow therapeutic index?

Accepted Answer

A drug is considered narrow therapeutic index (NTI) when the difference between its minimum effective concentration and its minimum toxic concentration is small — typically defined as a less than 2-fold difference between the median toxic dose (TD50) and the median effective dose (ED50), or a less than 2-fold difference between minimum toxic and minimum effective plasma concentrations. This means small changes in dose or formulation can shift patients from therapeutic to toxic or sub-therapeutic ranges.

Question 2

Why does switching between generic formulations of NTI drugs matter?

Accepted Answer

FDA bioequivalence standards allow AUC and Cmax to fall within 80–125% of the reference product. For most drugs this is clinically insignificant, but for NTI drugs with a 2-fold therapeutic window, a 25% difference in exposure can shift patients outside their therapeutic range. FDA applies tighter bioequivalence criteria (90–111% instead of 80–125%) for levothyroxine, warfarin, and some immunosuppressants.

Question 3

Which NTI drugs most often require therapeutic drug monitoring (TDM)?

Accepted Answer

The NTI drugs most commonly requiring TDM include: vancomycin (AUC/MIC-guided dosing), digoxin (narrow range 0.5–2 ng/mL), warfarin (INR monitoring rather than plasma levels), phenytoin (10–20 mg/L; non-linear PK), carbamazepine (4–12 mg/L), lithium (0.6–1.2 mmol/L), cyclosporine and tacrolimus (trough or AUC), and aminoglycosides.

Question 4

Does NTI status affect how a drug should be stored and dispensed?

Accepted Answer

Yes. NTI drugs with high potency risk (e.g., concentrated KCl, anticoagulants, insulin) require additional storage safeguards — segregated storage, restricted access, distinct labelling. For oral NTI formulations, unit-dose dispensing and barcode verification at bedside reduce administration errors. Pharmacies should track lot number and manufacturer when dispensing NTI generics to identify potential formulation switches.

Drug	Therapeutic range	Toxic threshold	Monitoring parameter
Warfarin	INR 2–3 (most) or 2.5–3.5 (mechanical valves)	INR >4 (bleeding)	INR
Digoxin	0.5–2 ng/mL	>2 ng/mL (arrhythmia, nausea)	Serum digoxin level
Phenytoin	10–20 mg/L (total)	>20 mg/L (nystagmus, ataxia)	Total + free level
Lithium	0.6–1.2 mmol/L	>1.5 mmol/L (tremor, confusion, renal)	12 h post-dose serum Li
Cyclosporine	Protocol-dependent trough (100–200 ng/mL typical)	Nephrotoxicity, hypertension	C₀ trough or AUC₀₋₁₂
Tacrolimus	5–15 ng/mL (transplant type/time dependent)	Nephrotoxicity, neurotoxicity	C₀ trough
Vancomycin	AUC/MIC ≥400 (≤600) mg·h/L	Nephrotoxicity (AUC >650)	AUC-guided (Bayesian)
Carbamazepine	4–12 mg/L	>12 mg/L (diplopia, ataxia)	Trough serum level
Levothyroxine	TSH 0.5–4 mIU/L	Suppressed TSH → atrial fibrillation, osteoporosis	TSH (+ free T4)

Narrow Therapeutic Index (NTI)

Definition

Common NTI Drugs — Ranges and Monitoring

Frequently asked questions