Agonists and antagonists !

Drugs that switch a receptor on, switch it partly on, or block it, and the different ways one drug can cancel another.

Updated 2026-11-01

An agonist binds a receptor and activates it; an antagonist binds without activating and prevents activation by others. Between them lie partial agonists and inverse agonists. How an antagonist blocks (competitively, irreversibly, at another site, or by acting on another system) changes whether more agonist can overcome it and how long the block lasts.

Types of agonist

Full agonist: high intrinsic efficacy
Partial agonist: intermediate efficacy; useful as a ceiling (less respiratory depression with buprenorphine) but can trigger withdrawal in people dependent on a full agonist
Inverse agonist: needed only where receptors are active without a ligand
Allosteric modulators bind elsewhere on the receptor and turn the natural transmitter's effect up or down (benzodiazepines on the GABA-A receptor)

Receptor antagonism

A competitive antagonist occupies the same site as the agonist and can be displaced. The rocuronium label describes it competing with acetylcholine at the motor end plate, which is why an anticholinesterase such as neostigmine, by raising acetylcholine, reverses the block. Naloxone displaces opioids from mu receptors; because it may wear off before the opioid, patients are watched for re-sedation. Irreversible antagonists bind covalently; their effect lasts until the body makes new protein.

Other ways to cancel a drug's effect

Chemical antagonism: one molecule binds and inactivates the other (protamine binds heparin; sugammadex encapsulates rocuronium)
Physiological (functional) antagonism: two drugs act on different receptors with opposite effects (adrenaline opposes histamine-driven bronchospasm and hypotension in anaphylaxis)
Pharmacokinetic antagonism: one drug lowers the level of another (an enzyme inducer)

Why it matters

Knowing the type of antagonism predicts the clinical answer: give more agonist to overcome a competitive block, wait for new protein after an irreversible one, or use an opposing system in an emergency. It also explains antidotes and reversal agents.

Test yourself

Think of the answer, then tap the question to see it.

Why can buprenorphine precipitate withdrawal in someone taking a full opioid agonist?

As a partial agonist with high affinity, it displaces the full agonist and produces a smaller effect, so overall receptor activation drops.

What happens to the agonist dose–response curve with a competitive antagonist?

It shifts to the right (lower apparent potency) but the maximum response can still be reached with more agonist.

How does neostigmine reverse a non-depolarising block such as rocuronium?

By inhibiting acetylcholinesterase it raises acetylcholine at the end plate, which outcompetes the competitive blocker.

What kind of antagonism does adrenaline provide against histamine in anaphylaxis?

Physiological antagonism: it acts on adrenergic receptors to produce effects opposite to histamine's, without touching histamine receptors.

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Example drugs

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: 15.4 Drugs that Affect the Autonomic System (OpenStax (Rice University), 2022); DailyMed - ROCURONIUM BROMIDE injection, solution (DailyMed (U.S. National Library of Medicine), 2026); 9.1 Signaling Molecules and Cellular Receptors (OpenStax (Rice University), 2018).

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