Many drugs are weak acids or bases and are, therefore, ionized to some extent in the medium in which they become concentrated in the body. In general, these compounds are more fat soluble in the nonionized form. Consequently, the nonionized form is more permeable to the cell membrane. It is important, therefore, to know the extent of ionization of the drug in the medium from which it is being absorbed. This will depend upon the pH of the medium and the pK of the drug. The pK of the drug is the negative logarithm of the equilibrium constant and is a constant for each drug under physiological conditions. Knowledge of the pK of the drug and the pH of the medium permits a determination of the ratio of ionized (ci) to unionized (cu) concentrations of the drug.