margin of safety

If the therapeutic index judges a drug by its two midpoints, the margin of safety asks a tougher, more honest question: how far apart are the dose that helps almost everyone and the dose that starts harming almost no one? It looks at the edges of the curves, not their centres.

A common formulation expresses the margin of safety as the dose range between the dose effective in most of the population (around the ED99) and the dose that begins to be toxic in a small fraction (around the TD1 or LD1). Because adverse outcomes matter most at the extremes, this is a more clinically meaningful gauge of overlap than the central ratio of the therapeutic index.

The margin of safety captures something the therapeutic index misses: the slope and spread of the curves. Two drugs with identical therapeutic indices can have very different margins if one has steep curves whose extreme effective and toxic doses nearly touch. A narrow margin of safety is the formal reason a drug needs cautious dosing, monitoring, or replacement by a safer alternative.