Drugs are substances that alter the biochemical processes in the body.

Drugs that have beneficial effects are used in medicines.

A medicine usually contains the drug plus other ingredients such as fillers to add bulk or sweeteners to improve the taste.

Drugs generally work by binding to specific protein molecules. These protein molecules can be found on the surface of a cell (receptor) or can be specific enzyme molecules within a cell.

Drugs that act on receptors can be classified as agonists or antagonists.

→ An agonist mimics the natural compound and binds to the receptor molecules to produce a response similar to the natural active compound.

→ An antagonist prevents the natural compound from binding to the receptor, and so blocks the natural response from occurring.

For example, salbutamol is an agonist which mimics adrenaline and beta blockers are antagonists which prevent adrenaline from acting by blocking its receptors.

Many drugs that act on enzymes are classified as enzyme inhibitors and act by binding to the active site of the enzyme and blocking the reaction normally catalysed there.

The overall shape and size of a drug is such that it interacts with a receptor binding site or binds to the active site of an enzyme. The types of interactions formed can include van der Waals forces and/or ionic bonds.

→ They also often include hydrogen bonding.

The structural fragment of a drug molecule that allows it to form interactions with a receptor binding site or to an enzyme active site normally consists of different functional groups correctly orientated with respect to each other.

By comparing the structures of drugs that have similar effects on the body, the structural fragment that is involved in the drug action can be identified.