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How are drugs classified and grouped? - ePharmacology

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Today's topic is about how drugs are classified. Let's start!

How Are Drugs Classified?

There are several thousands of drugs available for the treatment of diseases. So, it is not a easy task to classify them in a uniform manner. There are several ways to classify them:

  1. Chemical nature of the drug
  2. Disease condition/Pharmacotherapeutics
  3. Affected organ system
  4. Generation
  5. Receptor agonist and antagonist
  6. Drugs approved for marketing

1. Chemical nature of drug

Some drugs are classified purely on the chemical nature, for example, sulfonamides, benzodiazepines, catecholamines, barbiturates, thiazide diuretics and imidazoles.

Let's discuss some of the examples.


There are several thousands of sulfonamides of which clinically important are:

  • Sulfadiazines
  • Sulfadimidine
  • Sulfamethoxazole
  • Sulfasalazine

All the sulfonamides have a basic nucleus: p-aminobenzene sulfonamide.


The drugs like diazepam, nitrazepem, flurazepem belong to a group called benzodiazepines. Benzodiazepines refer to chemical compounds containing a benzene ring fused to a seven member diazepine ring.


The term catecholamine is derived from the catechol nucleus and 2 carbon side chains with a terminal amino group (ethylamine).

The catechol nucleus consists of a benzene ring and two hydroxyl groups. That is why, catechol nucleus of catecholamine is also called 3,4 dihydroxybenzene or m-p-dihydroxybenzene.

Hope you got the point about chemical classification of drugs.

However,Not all drugs are classified on the basis of chemical nature.

2. Disease Condition / Pharmacotherapeutics

Drugs may be classified on the basis of symptoms or diseases in which they are used.

This method of classification is either restrictive or broad.

For example, the drugs used for the treatment of various diseases such as hypertension, malaria, tuberculosis or epilepsy may be grouped by adding the prefix –anti-. We use the term anti-hypertensive, anti-malarial, anti-tubercular or anti-epileptic agents. These are example of restrictive classification.

Anti-viral, anti-bacterial or anti-cancer drugs are the examples of broad classification.

Aspirin or paracetamol is used to relieve pain. These belong to analgesics.

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In this method of classification, there are some drugs which are used in other diseases. For example, diuretic is used not only as anti-hypertensive drug but also in other diseases.


3. Affected Organ System

Drugs can also be classified on the basis of the organ system affected, such as drugs that act on the nervous system, gastrointestinal tract, and cardiovascular system.

Drugs that affect gastrointestinal function may be further categorized as drugs for the treatment of peptic ulcer, anti-diarrheal, laxatives, digestants, and anti-emetics.

4. Generation

Recently there is a tendency to classify some drugs (anti-microbial, oral sulfonylurea hypoglycemic agents, and retinoids) on the basis of generation: first, second, third, fourth generations.

Let me explain it to you by giving examples of cephalosporins (anti-bacterial).


Cephalosporins are classified-on the basis of general features of anti-microbial activity-into four generations.

First generation cephalosporins:

Drugs of first generation include:

  1. cefazolin
  2. cephalothin
  3. cephalexin
  4. cefadroxil

These drugs have good activity against gram positive bacteria. They have relatively modest activity against gram negative bacteria.

Second generation cephalosporins:

Second generation cephalosporins are:

  1. cefuroxime
  2. cefaclor
  3. cefoxitin
  4. cefamandole
  5. cefonicid
  6. cefotetan
  7. ceforanide

These drugs have increased activity against gram negative bacteria.

Third generation cephalosporins:

Drugs of third generation cephalosporin are:

  1. cefotaxime
  2. ceftrizoxime
  3. ceftazidime
  4. cefrodoxime
  5. ceftriaxone

These drugs are more active than the drugs of second generation against gram negative bacteria. They are less active than the drugs of first generation against gram positive bacteria, but they have the ability to cross the blood-brain barrier.

Similarly, oral sulfonylurea hypoglycemic agents are divided into 2 generations: first and second. Drugs of second generation (glibenclamide, flipizide, gliclazide) are considerably more potent than the first (tolbutamide, tolazamide, chlorpropamide).

Retinoids are classified into first, second, and third generation.

5. Receptor agonist and antagonist

Nowadays it is found that a lot of drugs produce their pharmacological effects by acting on their specific receptors.

On the basis of this concept, drugs may be classified as:

  1. receptor agonists: produces positive effect
  2. receptor antagonists: produces negative effect.

For example, dopamine receptor agonists are levodopa, bromocriptine, and apomorphine. Dopamine receptor antagonists are chlorpromazine, domperidone, pimozide, and haloperidol.

Like dopamine receptor agonist and antagonist, there are cholinoreceptor agonist and antagonist, adrenoceptor agonist and antagonist, histamine receptor agonist and antagonist, 5-HT receptor agonist and antagonist, etc.


6. Drugs approved for marketing

While prescribing a drug, it is classified into three groups:

  1. Over the counter (OTC) drug,
  2. Prescription only medicine (POM)
  3. Controlled drug

OTC drugs are those which can be prescribed without a prescription. There drugs are safer and can be self-medicated. It includes analgesics like paracetamol, antacids, anti-diarrheal agents and anti-septic drugs. The choice of OTC drugs is not strictly followed in the developing countries.

POM is a drug that requires a prescription from a licensed practitioner to be dispensed by a pharmacist (e.g. anti-histamines, anti-anxiety drugs, anti-malarial agents, and anti-biotics).

Controlled drugs are those drugs whose use and distribution is tightly controlled because of its abuse potential or risk (e.g. morphine, pethidine)

Self-Assessment Quiz!

For each question, choose the best answer. The answer key is below.

  1. Diazepine is a
    • 5 membered ring
    • 6 membered ring
    • 7 membered ring
    • 8 membered ring
  2. All of the following are 2nd generation cephalosporin except
    • Ceforanide
    • Cefoxitin
    • Cefadroxil
    • Cefamandole
  3. 2nd generation cephalosporins have the ability to cross the blood-brain barrier?
    • True
    • False
  4. OTC drugs are safer and can be self-medicated
    • True
    • False

Answer Key

  1. 7 membered ring
  2. Cefadroxil
  3. False
  4. True

Interpreting Your Score

If you got between 0 and 1 correct answer: You should re-read the article and then take the quiz again! Don't give up! Remember patience is the key to success!

If you got 2 correct answers: Good but you can do better! Perhaps you should read the article one more time!

If you got 3 correct answers: Well-done! But definitely you can do better! :)

If you got 4 correct answers: Genius!

Test Your Knowledge!

Answer the following questions:

1. How do you classify drugs?

2. What does catecholamine mean? Write down the chemical structure of catechol nucleus.

3. What does OTC mean? Name four OTC drugs.

5. What are the controlled drugs? Why are they called controlled drugs?

© 2013 Oyon B


Krubo Korvieh on October 16, 2016:

Thanks a million, I was able to know the classifications of drugs

Muhammad Waqas Khan Mughal from Mansehra on October 04, 2015:

Great job .!!!

Jessa Mae T. Gamlanga on September 25, 2015:

I have so much many learned thank you so much for everything!

isaac on December 04, 2014:

Thanks for your kind words

ankush Kumar on December 27, 2013:

This classify gastro intestinal agents and example are very easy and I learn mo're

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