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A receptor is a specific macromolecule, usually a protein, situated either in cell membranes or within the cell, to which a specific group of ligands, drugs or naturally occurring substances (such as neurotransmitters or hormones) can bind and produce pharmacological effects. There are three types of ligands: agonists, antagonists and partial agonists.

An agonist is a substance that stimulates or activates the receptor to produce an effect, e.g. salbutamol at the β₂‐receptor.

An antagonist prevents the action of an agonist but does not have any effect itself, e.g. losartan at the angiotensin II receptor.

A partial agonist stimulates the receptor to a limited extent, while preventing any further stimulation by naturally occurring agonists, e.g. aripiprazole at the D2 and 5‐HT1a receptors.

The biochemical events that result from an agonist–receptor interaction to produce an effect are complex. There are many types of receptors and in several cases subtypes have been identified which are also of therapeutic importance, e.g. α‐ and β‐adrenoceptors and nicotinic and muscarinic cholinergic receptors.