Cmax
Cmax is simply the highest plasma concentration a drug reaches after a dose — the peak of the curve when you plot blood level against time. After swallowing a tablet, the level climbs as absorption outpaces elimination, crests at Cmax, then falls as elimination takes over. It is the summit of the mountain on the concentration-time graph.
Cmax is an emergent result of the tug-of-war between absorption and elimination, not a property you set directly. It rises with a larger dose or faster absorption, and the time at which it occurs is called Tmax. For the same total exposure, a fast-absorbed formulation gives a tall, early Cmax, while a slow-release one gives a lower, flatter peak.
Clinically Cmax matters most for two opposite reasons. For drugs whose toxicity tracks the peak (such as the kidney and ear toxicity of aminoglycoside antibiotics), a high Cmax is dangerous. For others whose effect depends on hitting a high peak (such as the bacterial killing of those same aminoglycosides), a robust Cmax is therapeutically desirable.
Once-daily gentamicin is deliberately given as a single large dose to drive a high Cmax for maximal bacterial killing, while letting the trough fall low between doses to limit kidney toxicity.
High peak for efficacy, low trough for safety.
Cmax and Tmax are read straight off the concentration-time curve and are the two parameters bioequivalence studies compare for absorption rate.