Physicochemical Drug Interactions and Incompatibilities
This chapter deals with some practical consequences of the physical chemistry of drugs- particularly their interactions with each other, with solvents and with excipients in formulations. Sometimes the interaction is beneficial and sometimes not. In reading this chapter you should appreciate that there are several causes of interactions and incompatibilities such as: pH effects, change of solvent characteristics on dilution, cation-anion interactions, salting-out and salting-in, chelation, ion-exchange interactions, adsorption to excipients and containers, interactions with plastics, protein binding. Equations can be written to describe most of these, but these formulae can be applied in vivo only as a first-line calculation because of the complexity of the body. Nevertheless the equations are important to allow some prediction of the magnitude of effects.
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