TD50

TD50 is the dark twin of ED50. Where ED50 is the dose that helps half the population, TD50 is the dose that produces a specified toxic effect in half the population. It marks where benefit starts giving way to harm.

Like ED50, TD50 is read from a quantal curve, but the chosen endpoint is an adverse one — vomiting, tremor, a defined laboratory abnormality, or any pre-specified sign of toxicity. The midpoint dose at which half the group shows that toxicity is the TD50. Because toxicity endpoints can be mild or severe, a single drug has many possible TD50 values depending on which harm you measure.

TD50 matters because it anchors the upper boundary of safe use. The ratio TD50 divided by ED50 is the therapeutic index, the most common single-number estimate of how forgiving a drug is. In humans, for ethical reasons, TD50 is often estimated from non-lethal adverse-effect data or animal studies rather than measured directly.