Difference Between Agonist and Antagonist (With Table)

Pharmacology is a branch of medicine that establishes the relationship between chemical agents and the human body. These chemical agents can be both natural as well as synthetic. Agonist and Antagonist are two examples of these chemical agents studied under pharmacology having opposite effects on the receptors leading to different characteristics.

Agonist vs Antagonist

The main difference between an agonist and an antagonist is their response to the receptors. While an agonist has a similar response as the receptor, the antagonist inhibits the response of a receptor. Their names themselves hint at their opposite behavioral traits. They have contradictory functions that are in use to either activate or deactivate receptors.

An agonist is a chemical agent that has akin reactions to a receptor. They copy the functions of a neurotransmitter, thus leading to the same function as a receptor. They can be both natural and synthetic. Natural ones are hormones, and chemical ones are drugs. For example, the medication Succinyl Choline mimics the response of Acetylcholine.

An antagonist is a chemical agent that obstructs the response of a receptor. It has the opposite reaction as a neurotransmitter. As the name suggests, it is the opposite of an antagonist. Their basic function is to hinder the working of the receptors. As a result, they are also known as blockers. For example, the effect of curare on Acetylcholine.

Comparison Table Between Agonist and Antagonist

Parameters of ComparisonAgonistAntagonist
MeaningIt is a chemical substance that mimics the reactions of a receptor.It is a chemical substance that inhibits the reaction of a receptor.
FunctionIt aids the function of the neurotransmitter.It stops or slows the function of the neurotransmitter.
PotencyIts potency is inversely related to EC50 (effective concentration).Its potency is measured by the IC50 value (inhibitory concentration).
ResponseIt has the same response as a receptor.It has the opposite response as a receptor.
Side effectsToo much consumption of agonists leads to malignant syndrome.Side effects may include dizziness, drowsiness, headache, or dryness of the mouth or skin.
UsageIt is used to stabilize hypertension or panic attacks or aid in alcohol withdrawal.It is used to reverse the effects of drugs that have been already consumed.

What is an Agonist?

An agonist is a chemical substance that imitates the function of a receptor. They produce a similar action as a neurotransmitter. For example, Succinyl Choline shows the same effects as Acetylcholine at nicotinic acetylcholine receptors.

It can be classified into two types. One is an endogenous agonist, and another is an exogenous agonist. An endogenous agonist is a substance that is naturally produced in the body. These generally include hormones or neurotransmitters. Dopamine and serotonin are examples of endogenous agonists.

An exogenous agonist is a chemical substance that has been infused externally. They are the chemical drugs that are in use to activate a receptor. Synthetic dopamine is an example of exogenous agonists that mimics the response of dopamine.

The potency of an agonist is the amount of agonist required for a significant response. It is inversely related to the EC50 value. EC50 or 50% Effective Concentration is the concentration of agonist required to bring out half of the desired response.

Side effects of agonists are numerous. It varies with the dosage and period of chemical agonists consumed. They may include drowsiness, dizziness, nausea, vomiting, paranoia, etc. Overdose of a dopamine agonist may cause a malignant syndrome that can be life-threatening in some situations.

What is Antagonist?

An antagonist is a chemical agent that hinders or obstructs the response of a receptor. Its prime function is to block the biological response of a neurotransmitter. It performs the opposite function of an agonist.

They are also known as blockers due to their inherent characteristics. Examples include calcium channel blockers, alpha-blockers, or beta-blockers.

They can be of two types. One is natural, whereas the other is synthetic. Synthetic antagonists are external agents that are infused in the form of medication. Drugs like naloxone, naltrexone, and Quisqualic acid are a few such examples. 

Natural antagonists are produced in the body. They are common agents having the same effect as a chemical antagonist whereby stopping the function of the receptors. Asymmetric dimethylarginine is an example of a natural antagonist.

The potency of antagonists is measured by the IC50 value. IC50 or 50% Inhibitory Concentration is the amount of antagonist needed to inhibit half of the biological response produced by an agonist.

It may have some undesired side effects. These include abdominal pain, dizziness, coughing, difficulty in breathing, joint pain, inflammation of blood vessels, etc.

Main Differences Between Agonist and Antagonist

  1. An agonist produces a similar response whereas an antagonist hinders the response of the receptors.
  2. Their inherent functions are different. The former stimulates the function whereas the latter stops the desired function.
  3. Potency is the dosage required to bring forth the desired response. The potency of an agonist is inversely related to its EC50 value. On the other hand, the potency of an antagonist is related to its IC50 value.
  4. Side effects of agonists may be a malignant syndrome. On the other hand, that of an antagonist may include dizziness, drowsiness, headache, or dryness of the mouth or skin.
  5. Agonists are generally used to check panic attacks and hypertension. Whereas, antagonists are used in reversing the effects of drugs that are already consumed.
  6. There may exist some agonists that work as an antagonist. However, the reverse role is not possible.


The various branches of medicine have led the way to innovations. The study of pharmacology has been prevalent for years. It binds the relation between chemical agents and the human body.

Agonists and antagonists are terms used in this field of study. They show how a chemical agent reacts when in contact with a particular receptor.

The underlying etymology suggests that the two terms are opposite of each other. One mimics the response, whereas the other obstructs the response of the neurotransmitters.


  1. https://www.sciencedirect.com/science/article/pii/S0021925817348068
  2. https://www.mdpi.com/2079-7737/3/4/645