Structure-activity relationships (SAR) describe how the structure of related drugs influences their behaviour, for example whether they are agonists or antagonists. In order to understand how differences in drug structure can affect activity it is necessary to appreciate drug development methods and some basic organic chemistry. Once the properties of the contributing groups are understood, then it becomes easier to predict the likely behaviour of a drug molecule compared with the parent drug. In addition, knowledge of the structural properties of a drug may help us appreciate some of their physicochemical properties, such as their solubility in oil and water, their pKa values and whether they are weak acids or bases. These in turn help us understand the pharmacokinetic behaviour of a drug.

Drug design starts with a lead compound that has the required action in an animal model, but is not necessarily ideal; for example, the drug may resemble a neurotransmitter or be an enzyme inhibitor. By adding various functional groups to this compound it is possible to develop a more specific drug to target the required system. Once a compound with the most favourable pharmacodynamic effects is found, further modifications may be made to make the drug's pharmacokinetic behaviour more desirable.

In this chapter we will introduce some basic organic chemistry and identify the structures associated with drugs commonly used in anaesthesia. Those basic structures that should be readily identified are mentioned briefly below, together with diagrams of their structures and examples relevant to anaesthesia.