Agonist vs Antagonist: Difference and Comparison

Though the words agonist and antagonist differ in meaning depending on the context, these terms are more frequently used in pharmacodynamics.

Pharmacodynamics studies and deals with the effect of a drug on an organism. Not all drugs work along with the existing defence 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, medical experts choose and use agonist and antagonist drugs.

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 drugs 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 agonists like full agonists, co-agonists, selective agonists, inverse agonists etc.

The prefix in the term antagonist indicates the meaning. As the name suggests, antagonist drugs work opposite to agonist drugs. Antagonist drugs attach themselves to the receptors but oppose and block any biological receptor by obstructing it.

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

Comparison Table

Parameter of Comparison	Agonist	Antagonist
Definition	A 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.
Example	Methadone.	Naltrexone.
Function	Triggers the receptors.	Blocks the response to the receptors.
Application	Used in treatment to cure people of drug addiction.	It is also used in anti-drug addiction therapy.
Effect	It always assists in stimulating the receptors.	It blocks the reception.
Drawbacks	The body could develop tolerance; thereby, the drug’s efficacy gets reduced.	It could deteriorate the person’s improvement if the correct dosage is not given at the right time.

What is Agonist?

Agonists are chemicals that attach firmly to the target receptor and stimulate 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 triggered with the help of drugs, it is termed an exogenous agonist. Based on the activation and the level of the extent they create, agonists are classified under various categories.

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

Certain chemically created agonists, called super agonists, produce a greater effect than the natural agonist.  Certain agonist which works only on a unique receptor is called selective agonists. Buspirone is a good example of the category mentioned above.

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

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?

Normally, an antagonist refers to an object that opposes natural activity. The object could be a thing, person, substance etc. The term’s meaning can be derived from the first half of the word itself.

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

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

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

After getting attached to the receptor 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 become competitive antagonists 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.

Like 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 trigger 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. Some agonists could act as antagonists, but the reverse, i.e. antagonist working as an agonist, does not exist.
5. An improper dose of agonist could only increase the body’s tolerance, making it ineffective, whereas an improper dose of antagonist chemicals could create deteriorating effects.

References

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