**1. Introduction**

This chapter aims to elaborate on the linkages between pharmacokinetics and the advent of adverse effects of drugs. It is well known that pharmacokinetics is about the journey of the drug in the body, from its absorption, through its distribution and metabolism, to its elimination from the body. During this journey, after its absorption and distribution, the drug reaches its specific sites where it interacts with its receptors, usually proteins and enzymes, and produces its biological effects; this is known as "pharmacodynamics." The biological effects lead to clinical effects that are observed in patients; this is known as "therapeutics or pharmacotherapeutics." In the following sections, the actions of the body on a drug and the actions of the drug on the body are reviewed in each stage to explain how adverse effects occur. In doing so, the mechanisms and risk factors of adverse effects will be addressed. The next section deals with the absorption, followed by the distribution and excretion of drugs as they relate to the occurrence of adverse effects. A final section will deal with risk factors before some concluding remarks are presented.
