The ratio of the toxic dose to the effective dose (TD50/ED50) — a pharmacodynamic measure of a drug's safety margin. A narrow therapeutic index (TI ≈ 1–2) means the effective dose is close to the toxic dose, requiring careful dose titration and therapeutic drug monitoring.
Definition
The therapeutic index (TI) quantifies the separation between
the dose that produces the desired effect and the dose that produces toxicity:
TI = TD50 / ED50
Margin of safety = TD1 / ED99
Where TD50 is the dose producing toxicity in 50% of a
population and ED50 is the dose producing the desired
effect in 50%. The margin of safety is a more conservative measure,
using the 1st percentile of the toxicity distribution and 99th percentile of the
efficacy distribution — capturing the overlap of the two dose-response curves.
The therapeutic index is distinct from the
therapeutic window —
the concentration-based clinical guidance range (MEC to MTC) used in
TDM. TI is derived from population
dose-response data; the therapeutic window is applied to individual patients.
Narrow Therapeutic Range and Drug Safety
The therapeutic index is a cornerstone concept in pharmacology that determines how safely a drug can be prescribed. Drugs with a narrow therapeutic range — where the effective dose is dangerously close to the toxic dose — present significant clinical challenges. The therapeutic window for such drugs is so tight that even minor changes in absorption, metabolism, or excretion can push a patient from efficacy into toxicity or therapeutic failure.
Five drugs are frequently cited as the most clinically important narrow therapeutic index agents: warfarin, phenytoin, digoxin, lithium, and aminoglycosides. Each of these drugs requires careful dose titration, routine therapeutic drug monitoring (TDM), and heightened awareness of drug-drug interactions. According to the NCBI StatPearls review on therapeutic drug monitoring, NTI drugs account for a disproportionate share of preventable adverse drug events in hospital settings.
The European Medicines Agency (EMA) and FDA both impose tighter bioequivalence requirements on NTI generic drugs, recognising that standard 80-125% confidence intervals may be insufficiently protective for patients on these agents. For a dedicated discussion, see our narrow therapeutic index glossary entry.
Narrow Therapeutic Index (NTI) Drugs
NTI drugs require careful clinical management because small dose changes or
PK variability (disease, interactions, genetics) can push levels from therapeutic
to toxic. Key examples:
Lithium: therapeutic 0.6–1.0 mmol/L; toxic >1.5 mmol/L.
NSAIDs, thiazides, and ACE inhibitors ↑ levels by reducing renal elimination.
See our
psychiatric medication guide.
Phenytoin: therapeutic 10–20 mg/L; nonlinear (Michaelis-Menten)
kinetics mean small dose increments above 300 mg/day cause disproportionate
level increases. See nonlinear PK glossary.
Warfarin: no fixed concentration range — monitored by INR (target
2.0–3.0 for most indications). CYP2C9 and VKORC1 genotypes are the primary
determinants of dose requirements. See our
anticoagulant therapy guide.
The therapeutic index (TI) is the ratio of the median toxic dose (TD50 — the dose producing toxicity in 50% of a test population) to the median effective dose (ED50 — the dose producing the desired effect in 50%). TI = TD50 / ED50. A high TI (e.g., >10) indicates a wide safety margin between effective and toxic doses. A low TI (e.g., <2) indicates a narrow margin where effective doses approach toxic doses — requiring careful dose titration and therapeutic drug monitoring. The TI is a pharmacodynamic concept derived from dose-response experiments, typically in animal models during drug development, and is distinct from the therapeutic window (which refers to the plasma concentration range in patients).
How does the therapeutic index differ from the therapeutic window?
The therapeutic index (TI) is a ratio — TD50/ED50 — derived from population dose-response curves in drug development, expressing how much more drug is required to produce toxicity than efficacy. The therapeutic window (or therapeutic range) is a concentration range in individual patients — the plasma drug concentration between the minimum effective concentration (MEC) and the minimum toxic concentration (MTC) — used to guide dosing and TDM decisions. A narrow therapeutic index drug has clinical importance because inter-patient variability in PK (CYP polymorphisms, renal/hepatic function, protein binding, drug interactions) means the effective dose for one patient may produce toxicity in another. The therapeutic window is the clinical application of the TI concept to individual patient management.
Which drugs have a narrow therapeutic index and why does this matter?
Narrow therapeutic index (NTI) drugs are those where small differences in dose or plasma concentration lead to clinically significant toxicity or loss of efficacy. Examples include: digoxin (TI ~2; therapeutic 0.5–2 ng/mL; toxic >2 ng/mL — arrhythmias, GI toxicity); lithium (TI ~2; therapeutic 0.6–1.0 mmol/L; toxic >1.5 mmol/L — neurological, renal); phenytoin (saturable kinetics compound the narrow TI problem — see nonlinear PK); warfarin (highly variable PK via CYP2C9 genotype and multiple DDIs; INR range 2.0–3.0 = therapeutic window equivalent); cyclosporin, tacrolimus, aminoglycosides, vancomycin, methotrexate. NTI drugs typically require TDM, and FDA/EMA regulatory policies for NTI generics require tighter bioequivalence criteria (80–125% standard narrowed to 90–111.11% for some NTI drugs).
What drugs have a narrow therapeutic range?
Drugs with a narrow therapeutic range include warfarin (INR-monitored anticoagulant with extensive CYP2C9 and VKORC1 genetic variability), phenytoin (nonlinear Michaelis-Menten kinetics; therapeutic 10-20 mg/L), digoxin (TI ~2; therapeutic 0.5-0.9 ng/mL for heart failure), lithium (therapeutic 0.6-1.0 mmol/L; nephrotoxic above 1.5 mmol/L), aminoglycosides (concentration-dependent killing but ototoxic and nephrotoxic at sustained high levels), vancomycin (AUC/MIC-guided dosing), cyclosporine, tacrolimus, carbamazepine, and levothyroxine. All require therapeutic drug monitoring (TDM) or surrogate marker monitoring (INR for warfarin, TSH for levothyroxine) to maintain safe and effective therapy.
What is the margin of safety and how does it relate to the therapeutic index?
The margin of safety is a related but more conservative measure: TD1/ED99 — the dose toxic in 1% of the population divided by the dose effective in 99%. This avoids the overlap problem: even with TI >1, the dose-response curves for efficacy and toxicity may overlap if population variability is high, meaning some individuals will experience toxicity at doses that are sub-effective in others. The margin of safety (using 1st and 99th percentile instead of medians) captures this overlap. A drug with TI = 3 but high pharmacokinetic variability may have a much smaller margin of safety than a drug with TI = 2 but low variability. This is why inter-patient variability (CV%) in PK parameters is critical information for NTI drug management.