Therapeutic Window

The therapeutic window is the band of exposure between the minimum concentration that produces the wanted effect and the concentration at which harm becomes unacceptable. It is a clinical range rather than a laboratory constant, and it differs from the therapeutic index, which is a ratio derived from dose-response curves in a defined population. A window can be narrow because the toxic threshold is low or because the effective threshold is high, and the remedies for the two differ.

Warfarin, digoxin, lithium and the aminoglycosides are the standard narrow-window examples, and all of them attract routine concentration monitoring. Insulin has a narrow window for an instructive reason: hypoglycaemia is the intended pharmacology carried too far, not an off-target effect, so no amount of receptor selectivity widens it. Incretin analogues, by contrast, are limited chiefly by gastrointestinal tolerability, which is why they are escalated slowly rather than monitored by assay.

Window width decides the whole approach to dosing. A wide window permits fixed doses and titration to clinical response; a narrow one justifies therapeutic drug monitoring, adjustment for organ function, and formulations designed to lower the peak. It also determines how much a manufacturing deviation matters, since a product delivering ten percent more than labelled is trivial in one case and dangerous in the other.

The claim to distrust is that peptides have wide windows because they are endogenous or natural in origin. Windows are empirical and compound-specific, and on-target toxicity is common precisely among endogenous signalling molecules. A second failure is quoting an index derived from rodent lethality as though it described a human window; lethal dose ratios say nothing about the concentration at which a person stops tolerating a drug.