Tmax

If Cmax tells you how high the blood-concentration wave rises, Tmax tells you when the crest arrives — the time, measured from dosing, at which the plasma concentration reaches its maximum. A short Tmax means the drug peaks quickly (fast onset), a long Tmax means it builds up slowly.

Tmax is read directly from the concentration-time curve as the time of the highest measured point. It is governed mainly by the rate of absorption relative to elimination: a rapidly dissolving, rapidly absorbed drug peaks early, while a slowly releasing formulation or slow absorption delays the peak. At Tmax, the rate of drug entering the blood momentarily equals the rate of its removal, so the curve is flat at the top.

Tmax is a useful marker of absorption speed and onset of action, and it is one of the parameters compared in bioequivalence work, though it is treated more loosely than Cmax and AUC because it is a discrete value tied to sampling times rather than a smooth area. Formulation strongly shapes it: an immediate-release tablet may give a Tmax of an hour or two, whereas an extended-release version deliberately pushes Tmax later to prolong the effect.