A1-Introduction are antidepressants, cardiovascular drugs, anti-asthmatics, and anti-ulcer second, drugs can be classified depending on whether they act on a particular biochemical process. For example, antihistamines act by inhibiting the action of the inflammatory agent histamine in the body. Although this classification is more specific than the above, it is still not possible to identify a common feature relating all antihistamines. This is because there are various ways in which the action of histamine can be inhibited. Other examples of this kind of classification are cholinergic or adrenergic drugs a third method of classifying drugs is by their chemical structure (Fig. 2) Drugs classified in this way share a common structural feature and often share a similar pharmacological activity. For example, penicillins all contain a B-lactam ring and kill bacteria by the same mechanism. As a result, this classification can sometimes be useful in medicinal chemistry. However, it is not foolproof. Sulfonamides have a similar structure and are mostly antibacterial. However, some sulfonamides are used for the treatment of diabetes. Similarly steroids all have a tetracyclic structure, but the pharmacological effect of different steroids can be quite different. H CHa CO2H Steroids Fg. 2. Drugs classified by structure Finally, classifying drugs according to their molecular target is the most useful classification as far as the medicinal chemist is concerned since it allow a rational comparison of the structures involved. For example, anticholin esterases are compounds that inhibit an enzyme called acetylcholinesterase They have the same mechanism of action and so it is valid to compare the various structures and identify common features