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What are agonists and antagonists?

Updated on October 16th, 2022

Save up to 88% on your medications

Written and medically reviewed by Dorcas Morak, Pharm.D

Aren't you curious to know how your medications can elicit the targeted outcome? How can antihypertensive medications help you lower your blood pressure? How do erectile dysfunction medications revive your erection and vitality? How do opioids numb your pains? And how can antidepressants elevate your mood?

Drugs can either be an agonist or an antagonist based on how they interact with the receptor and you will find it amazing to learn how they can produce therapeutic results. But how do antagonists differ from agonist drugs?

What are receptors?

Receptors are protein molecules present on the cell surfaces used to communicate chemical information across the cell. The action of the ligand on the receptors regulates the observed response.

Ligands are chemicals that bind to receptors to trigger the cell to respond. Hormones, neurotransmitters, and medications are examples of ligands, which are specific against the receptor in action. They can be agonists or antagonists.

What are agonists and antagonists?

When an antagonist binds to a receptor, it inhibits the normal chemical response of the receptors on the cell while an agonist activates the receptors and triggers a response from the cell. In another word, the agonist turns on the receptor while the antagonist turns it off.

How does Agonist Drugs Work?

Endocrine and neurotransmitter systems are examples of endogenous agonists. Agonist drugs chemically mimic the endogenous agonist and activate the corresponding receptors when they bind to their active site. For instance, endogenous agonists like endorphins activate opioid receptors to relieve pain. Opioid painkillers such as morphine and hydrocodone bind to the same opioid receptor to produce the same effects. Another illustration is the hormone epinephrine, which binds to adrenergic receptors to cause a sympathetic response such as increased heart rate, and elevated blood pressure. Adrenaline, however, is a medication that causes a similar response via the same receptor and is used for cardiac arrest.


An agonist can be classified into three types:

Full Agonists

These agonists trigger a full response by binding directly to the receptor at the same active site as the endogenous ligands. They are fast in action. Examples are morphine for pain, codeine for pain, and clonidine for hypertension

Partial Agonists

These have both agonistic and competitive antagonistic effects in the absence and presence of a complete agonist respectively. In the presence of an agonist, they compete for the active site of the receptors resulting in a net reduction in the receptor activation seen with the full agonist by itself. Examples are buspirone for anxiety disorders, aripiprazole for schizophrenia, buprenorphine for opioid dependency, and nalmefene for opioid overdose.

Inverse Agonists

Inverse agonists bind to receptors and have the opposite effect of what is anticipated. In other words, they generate a lower response than the receptor's basal activity. Examples are antihistamines at H1 and H2 receptors.

How does Antagonist Drugs Work?

The antagonist blocks the receptor and prevents the agonist from activating it. For instance, clonazepam used for seizures is potentially harmful at high doses. Its overdose can be treated with flumazenil which blocks the action of the drug on the GABA receptors.


There are three classes of antagonist drugs:

Competitive Antagonists

These medicines compete with the agonist for the receptor's active site. The active site is successfully occupied by the compound (agonist or antagonist) with a higher concentration. For instance, if the antagonist is present in a larger concentration, it blocks the activating effect of the agonist. Examples include naloxone, an opioid receptor antagonist used to treat opioid overdose, and flumazenil, a competitive antagonist of benzodiazepines used to treat benzodiazepine overdose.

Noncompetitive Antagonists

These medications bind to the receptor's allosteric site, which prevents the agonist from activating the receptor by altering the structure of the receptor's active site. Non-competitive antagonism is not concentration-dependent. An example is ketamine, an anesthetic agent. It is a non-competitive antagonist on the NDMA receptor.

Irreversible Antagonists

Drugs known as irreversible antagonists bind covalently and permanently inhibit the receptor. Examples include aspirin for pain, omeprazole for peptic ulcer disease, and phenelzine for depression.

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