Therapeutic index

The ratio between a drug’s toxic dose and its effective dose. It estimates the separation between doses that cause harm and doses that produce a desired effect.

Median toxic dose: The dose that produces a specified toxic effect in half of a tested population. It supplies the toxicity value in the classic therapeutic-index calculation.
Digoxin: A cardiac glycoside used to treat selected heart conditions, including atrial fibrillation and heart failure. Its narrow margin between therapeutic and toxic exposure makes careful dosing essential.
Margin of safety (pharmacology): A pharmacological measure comparing doses that produce toxicity in nearly all subjects with doses effective in nearly all subjects. It uses extreme population-response doses rather than the median doses in the classic index.
Dose-limiting toxicity: An adverse effect severe enough to prevent further dose escalation in a treatment regimen. The toxicity endpoint chosen can change the practical safety margin.
Median effective dose: The dose that produces a specified therapeutic effect in half of a tested population. It supplies the efficacy value used as the denominator in the classic ratio.
Lithium (medication): A mood-stabilizing medication used primarily to treat bipolar disorder. Its narrow therapeutic range makes blood-level monitoring central to safe treatment.
Therapeutic window: The range of drug concentrations or doses that produce useful effects without unacceptable toxicity. It describes a usable range, while the therapeutic index is a ratio of dose measures.
Interindividual variability: Differences in biological responses among individuals exposed to the same drug or dose. A population ratio may conceal patients whose toxic doses are unusually low.
Dose-response relationship: The relationship between the amount of an exposure and the magnitude or probability of its effect. The index compares dose levels read from toxic and therapeutic response curves.
Warfarin: An anticoagulant medication that reduces blood clot formation by inhibiting vitamin K-dependent clotting factors. Small dose changes can shift treatment between inadequate anticoagulation and dangerous bleeding.

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