Therapeutic Index Calculator

The therapeutic index (TI) is a quantitative measurement of the relative safety of a drug. It compares the amount of drug that causes therapeutic effects to the amount that causes toxic effects, calculated as TI = TD50/ED50.

Therapeutic index is calculated using the formula TI = TD50 ÷ ED50, where TD50 is the median toxic dose (dose causing toxicity in 50% of subjects) and ED50 is the median effective dose (dose producing therapeutic effect in 50% of subjects).

A higher therapeutic index indicates greater drug safety. It means there's a larger margin between the effective dose and toxic dose, so patients would need to take much higher doses to reach toxic levels.

Drugs with a therapeutic index less than 2 are generally considered narrow therapeutic index drugs. These medications require careful monitoring as small changes in dose can lead to therapeutic failure or toxicity.

Therapeutic index helps assess drug safety during development and clinical use. It guides dosing recommendations, identifies drugs requiring therapeutic monitoring, and helps determine appropriate safety margins for patients.

TD50 is the dose causing toxicity in 50% of subjects, while LD50 is the lethal dose killing 50% of subjects. TD50 is often used for human therapeutic index calculations as it focuses on toxicity rather than lethality.

Unit conversion is important for accurate calculation. Both TD50 and ED50 must be in the same units before calculating the ratio. The therapeutic index itself is unitless as it's a ratio.

Examples include warfarin, digoxin, lithium, phenytoin, and theophylline. These drugs require careful dose monitoring and regular blood level checks to maintain efficacy while avoiding toxicity.