Difference Between Agonist and Antagonist

Though the word agonist and antagonist differ in their meaning depending on the context, these two terms are more frequently used in the field of pharmacodynamics.


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Pharmacodynamics studies and deals with the effect of a drug on an organism. Not all the drugs work along with the existing defense mechanism of the body when consumed.

Some drugs are designed to produce an opposing reaction when the drug binds with the receptor to achieve the desired result. Depending on the treatment, agonist drugs and antagonist drugs are chosen and used by the medical experts.

Key Takeaways

  1. An agonist is a substance that activates a receptor and produces a physiological response, while an antagonist is a substance that blocks the receptor and prevents the physiological response.
  2. Agonists are commonly used in medicine to stimulate a desired response in the body, while antagonists treat conditions where an overactive response needs to be suppressed.
  3. Examples of agonists include adrenaline and dopamine, while antagonists include beta-blockers and antihistamines.

Agonist vs Antagonist

The difference between agonist and antagonist drugs is in the way they function to produce the effect. While agonist drugs produce a specific action, antagonist drugs oppose a specific action.

Agonist vs Antagonist

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Agonist drugs are those which trigger certain cells in the brain to send a signal to the sensory nerve to activate some action or response. The neurotransmitters act as a chemical messenger to the receptors to produce a response.

It is not just neurotransmitters that activate the receptors. Exogenous agonists too induce the receptors for the desired response. There are different types of agonists like full agonists, co-agonists, selective agonists, inverse agonists etc.

The prefix in the term antagonist clearly indicates the meaning. As the name suggests, antagonist drugs work opposite to that of agonist drug. Antagonist drugs too attach themselves to the receptors but oppose and blocks any biological receptor by obstructing the receptor.

Antagonist drugs inhibit the regular action of the receptor. As they block the response, they are referred to as blockers.

Comparison Table

Parameter of ComparisonAgonistAntagonist
DefinitionA chemical that attaches to a specific receptor and triggers to induce a response.A chemical that attaches to a receptor but blocks and thwarts the natural response.
FunctionTriggers the receptors.Blocks the response to the receptors.
ApplicationUsed in treatment to cure people of drug addiction.It is also used in anti-drug addiction therapy.
EffectIt always assists in stimulating the receptors.It usually blocks the reception.
DrawbacksThe body could develop tolerance, thereby the efficacy of the drug gets reduced.Could deteriorate the person’s improvement if the correct dosage is not given at the right time.

What is Agonist?

Agonists are those chemicals which attach firmly to the target receptor and stimulates in inducing a natural response. It binds to the desired receptor and activates it. This activation can be done either by neurotransmitters or hormones as in the case of endogenous agonists.

If it is triggered with the help of drugs, then it is termed as exogenous agonists. Based on the activation and the level of the extent they create; agonists are classified under various categories.

If the level of activation occurs naturally in the body, it is termed as endogenous agonists. Paracetamol is an example for irreversible agonist that attaches permanently to the receptor and produces the chemical reaction.

There are certain chemically created agonists called super agonists which produces a greater effect than the natural agonist.  Certain agonist which works only on a unique receptor is called as selective agonists. Buspirone is a good example of the above-mentioned category.

Agonists are used in anti-drug addiction therapy to help people to come out of the vicious cycle of drugs. Agonists work by triggering the reception of opiate. Therefore, it helps people to stop cravings and alleviates it.

Half maximal effective concentration (EC50) is the index to measure the potency of agonist drugs. The lower the EC50 value, the stronger its effect would be. This also means the lower concentration of the drug induces the maximum response for an agonist whose EC50 value is extremely low.


What is Antagonist?

In the normal context, an antagonist is used to refer an object which opposes the natural activity. The object could be a thing, person, substance etc. The meaning for the term can be derived from the first half of the word itself.

As the name suggests, it is just opposite to agonist. In the field of pharmacodynamics, antagonist drugs refer to certain chemicals which thwarts the natural action of the response of the receptors after getting attached to it.

The antagonist is also referred to as blockers as they obstruct in the natural work of the receptors.

There are many types of an antagonist such as silent, competitive, uncompetitive and non-competitive. If it attaches to the same binding site without activating the receptor, then it is called a competitive receptor.

After getting attached to the receptor, it drives away the agonist present if any and blocks it. There are other agonists which act as an antagonist under special circumstances. For instance, partial agonists turn into competitive antagonist when a full agonist is present.

Similar to agonists, the effect of an antagonist can be understood by knowing its potency. Half maximal inhibitory concentration is the measure to describe the potency of antagonists. It is denoted as IC50.

Similar to EC50, the potency is higher when the IC50 value is lower. An example of an antagonist drug includes naloxone. Antagonist drugs are also used in anti-drug therapy.


Main Differences Between Agonist and Antagonist

  1. Agonist always produces a specific action while antagonist tries to block or oppose certain action or response.
  2. Agonists always induce or triggers the receptors for a certain natural response while antagonist tries to displace the agonist and blocks its path to the receptors.
  3. EC50 is the index used to know the effect of agonist drugs while IC50 is the index used to determine the potency of antagonist drugs
  4. There are some agonists that could act as an antagonist, but the reverse, i.e. antagonist working as agonist does not exist.
  5. An improper dose of agonist could only increase the tolerance of the body, thereby making it ineffective; whereas an improper dose of antagonist chemicals could create deteriorating effects.
  1. https://bpspubs.onlinelibrary.wiley.com/doi/abs/10.1111/j.1476-5381.1977.tb07532.x
  2. https://www.researchgate.net/publication/244874517_Receptors_agonists_and_antagonists
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