Clearly, side effects, interactions and allergies are a serious problem, and this chapter addresses the known biology behind some of the most important and most common drug side effects, drug interactions and allergies. Side effects are known to involve almost every body system, but some systems are more affected than others. The most likely effect would be with the system responsible for the drug's entry into the body. Pharmacology is the study of drugs, but this is an umbrella term incorporating several more specific sciences, the main ones being pharmacokinetics, pharmacodynamics and pharmacotherapeutics. There are four stages in pharmacokinetics: absorption, distribution, metabolism and elimination. Drug side effects, and particularly drug interactions, can occur at any of these stages. Drug allergies can be mild or serious, and even fatal. There are four allergic responses possible when drugs cause allergic reactions: type I, type II, type III and type IV.