An antagonist is a chemical that binds to a receptor but does not activate it. By occupying the site it prevents the natural transmitter or an agonist drug from producing its usual effect.

Competitive antagonists vie for the same binding site as the agonist; their effect can be overcome by increasing the concentration of agonist. Non-competitive antagonists bind elsewhere and change the receptor’s shape so that activation is impaired regardless of agonist concentration. Naloxone is an opioid antagonist used to reverse overdose; beta-blockers are antagonists at certain adrenaline receptors and are used to lower blood pressure and heart rate; many antipsychotic medications block dopamine receptors.

Antagonists are invaluable research tools as well as therapeutic agents. By selectively blocking a receptor, investigators can deduce the normal function of the transmitter system. Side effects arise when the same receptor serves multiple roles or when the drug is not perfectly selective.

Where an agonist turns a system on, an antagonist holds the switch in the off position.