Wikipedia
C is a term used in pharmacokinetics that usually refers to the minimum blood plasma concentration that a drug achieves in tested area after the drug has been administrated and prior to the administration of a second dose. C is the opposite of C, which is the maximum concentration that a drug achieves after dosing. C has direct relationships with other minimum concentrations such as the minimum inhibitory concentration (MIC) and is required to be higher than those concentrations to achieve the minimum efficacy.
In most cases C is directly measurable. At steady state the minimum plasma concentration can also be calculated using the following equation:
$C_{p min}= \frac{SFD k_a}{V_d(k_a-k)}\times\{\frac{e^{-k\tau}}{1-e^{-k\tau}}-\frac{e^{-k_a\tau}}{1-e^{-k_a\tau}}\}$
C is also an important parameter in bioavailability and bioequivalence studies, it is part of the pharmacokinetic information recommended for submission of investigational new drug applications.