Cmax (and Cmin)

C_max is the maximum or peak concentration of drug reached in the plasma following a dose of the drug. C_max may refer to the peak concentration achieved after a single dose, or to the peak concentration that follows each separate dose in a repeated dosing regimen, at steady state.

Many drug properties and patient characteristics impact the value of C_max. The size of the dose and route of administration, including the formulation of the drug (e.g. oral tablet versus sustained release capsule), as well as the rate and extent of absorption coupled with the rate and extent of distribution and the clearance rate all contribute to the value for C_max. Not surprisingly, for a particular drug at a given dose, IV bolus injection leads to the highest attainable C_max value which occurs very shortly after injection, since no absorption is necessary and the impact of distribution and elimination are minimised. The time following administration at which C_max occurs is referred to as t_max.

C_min refers to the trough concentration of drug in the plasma during a repeated dosing regimen, i.e. the drug concentration immediately prior to the administration of a subsequent dose. The C_max:C_min (or peak:trough) ratio in a repeated dosing regimen is determined by the drug’s half-life of elimination and by the chosen dosing interval, with shorter intervals leading to smaller peak:trough fluctuations. This point is discussed in more depth under the manual entry for repeated dosing calculations.