Agonist vs Antagonist – Difference and Comparison

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 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 mimic dopamine’s response.

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 agonist concentration 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 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.

Difference 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 used to check panic attacks and hypertension. Whereas, antagonists are used in reversing the effects of already consumed drugs.
  6. There may exist some agonists that work as an antagonist. However, the reverse role is not possible.

Comparison 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, panic attacks, or alcohol withdrawal.It is used to reverse the effects of drugs already consumed.