Calculator
Therapeutic Index Calculator
How much room there is between an effective dose and a toxic one.
Therapeutic index
Enter ED50 and TD50 to see the therapeutic index.
Dose values
Both must be in the same units.
Dose producing the desired effect in half the population.
Dose producing toxicity in half the population.
Try a known drug
Monitoring guide
Therapeutic index (TI) = TD50 ÷ ED50
ED50 is the dose that produces the desired effect in 50% of a population; TD50 is the dose that produces toxicity in 50%. Their ratio describes how much margin for error a drug allows.
A large TI means a dosing mistake is unlikely to cause harm. A small one — digoxin, warfarin, lithium — means the effective and toxic doses are close together, which is why those drugs need therapeutic drug monitoring.
Common questions
There is no formal cut-off, but above 10 is generally treated as a comfortable margin and below 2 as dangerous. Regulators tend to describe a drug as narrow-therapeutic-index when small changes in dose or blood level produce serious changes in effect.
The therapeutic index is a ratio of two doses derived from population curves. The therapeutic window is the range of actual plasma concentrations that are effective without being toxic — that is what therapeutic drug monitoring measures in a real patient.
ED50 is the dose producing the desired effect in 50% of the population; TD50 is the dose producing toxicity in 50%. Both come from fitting dose–response curves, and both must be expressed in the same units for the ratio to mean anything.
The older definition used LD50, the lethal dose in 50% of animals, giving TI = LD50 ÷ ED50. TD50 is preferred in human pharmacology because the clinically relevant boundary is toxicity, not death.
Arithmetically yes, if the toxic dose is lower than the effective one — which is why some otherwise effective compounds never become medicines. Certain cytotoxic drugs run close to this and are only justified by the severity of the disease.
Prepared by the PharmaWallah student team. Verify results with your textbook or instructor. Report an error · Editorial policy