Therapeutic Index

The therapeutic index is a ratio of two points on two dose-response curves: the dose producing toxicity in half of subjects divided by the dose producing the intended effect in half of subjects. In animal work the numerator is often the median lethal dose. A more honest variant, the certain safety factor, compares the tails instead, dividing the dose that harms one percent by the dose that works in ninety-nine percent, because medians ignore how steep or flat the two curves are.

Drugs with a narrow index are managed differently for that reason alone. Warfarin, digoxin, lithium, levothyroxine and phenytoin sit in a group where the gap between minimum effective and minimum toxic concentration is roughly twofold or less, which is why they attract therapeutic drug monitoring and tighter bioequivalence limits for generic substitution than ordinary products face.

Where the index sits determines how much variability a regimen can absorb. A wide index tolerates erratic absorption, missed doses and shifting renal function without consequence; a narrow one makes a formulation change, a new interacting drug or an episode of dehydration clinically significant. It is also toxicity-specific, so a compound can be reassuringly wide against acute harm and unacceptably narrow against a chronic effect such as axis suppression.

The number is routinely abused in research-chemical listings, where a rodent median lethal dose is quoted as a human safety margin. Acute lethality in a rodent measures one thing over a few days; it says nothing about chronic on-target consequences, nothing about receptor pharmacology that differs between species, and nothing about the effective dose that belongs in the denominator, which for most such compounds has never been established in humans at all.