In Section 4.6 (drug administration), first c0 is not same as second usage

The constant c_0 is used at the beginning of Section 4.6 as a concentration breakpoint in the (static) relationship between the concentration of an injected drug and its effect, but later c_0 represents the initial concentration of the drug. Should we change the notation here? (Perhaps check to see if there is some other variable that is commonly used?)