Cmax
Cmax is simply the highest the drug level ever gets in the blood after a dose. Picture plotting the drug's blood concentration over time after a tablet: the line climbs as the drug is absorbed, reaches a summit, then descends as elimination wins out. Cmax is the height of that summit.
It emerges from a tug-of-war between absorption pulling the concentration up and elimination pulling it down; the peak occurs at the moment these rates balance. A faster-absorbed or larger dose tends to give a higher Cmax, while slow-release formulations deliberately flatten and lower the peak to smooth out blood levels.
Cmax matters for both safety and efficacy. Many side effects are driven by the peak — too high a Cmax can push a drug past its safe ceiling — while for some drugs a high peak is needed to be effective. It is reported together with Tmax, the time at which the peak occurs, and the two are standard endpoints in pharmacokinetic and bioequivalence studies.
Converting an immediate-release painkiller to an extended-release form lowers Cmax and pushes Tmax later, trading a sharp early peak for steadier, longer-lasting levels and often fewer peak-related side effects.
Extended release trades a high peak for steadier levels.
Cmax describes the height of the peak, not the total exposure; two formulations can share the same AUC yet have very different peaks, which is why bioequivalence checks both.