What is the primary difference between a 'drug' and a 'medicine'?

A 'drug' is a chemical substance that, when administered, produces a biological effect on a living organism. This effect can be beneficial or harmful. A 'medicine,' on the other hand, is a drug or a combination of drugs that is used for the diagnosis, treatment, prevention, or cure of a disease, or to promote well-being. Essentially, all medicines are drugs, but not all drugs are medicines. For example, illicit substances like cocaine are drugs because they alter body function, but they are not considered medicines due to their harmful effects and lack of therapeutic use.

How do drugs interact with their molecular targets in the body?

Drugs interact with specific macromolecules in the body, known as molecular targets, through various types of chemical bonds. These targets are typically proteins (like enzymes, receptors, ion channels), but can also be nucleic acids, lipids, or carbohydrates. The drug molecule fits into a specific binding site on the target, much like a key in a lock. This binding can be reversible (e.g., hydrogen bonds, ionic bonds, van der Waals forces) or irreversible (covalent bonds), leading to a change in the target's function, which in turn produces a biological response. This interaction is highly specific, ensuring the drug acts where intended.

Explain the concept of 'agonist' and 'antagonist' in drug action.

In the context of drug-receptor interactions, an 'agonist' is a drug that binds to a receptor and activates it, mimicking the action of the body's natural chemical messenger (like a hormone or neurotransmitter). It produces a biological response. For example, salbutamol is an agonist for $eta_2$-adrenergic receptors, causing bronchodilation. An 'antagonist,' conversely, binds to a receptor but does not activate it. Instead, it blocks the binding of the natural messenger or an agonist, thereby preventing a biological response. Antagonists essentially 'block' the receptor. For example, cimetidine is an antagonist for histamine $ ext{H}_2$ receptors, reducing stomach acid.

What is the significance of classifying drugs based on their chemical structure?

Classifying drugs based on their chemical structure groups together compounds that share similar structural features, such as specific functional groups or ring systems. This approach is valuable because drugs with similar structures often exhibit similar pharmacological activities or mechanisms of action. It helps medicinal chemists understand structure-activity relationships (SAR), which are crucial for designing new drugs with improved efficacy and reduced side effects. While not always perfectly predictive, structural similarity often provides a strong starting point for drug discovery and development.

Why are some antihistamines sedating while others are non-sedating?

The difference between sedating (first-generation) and non-sedating (second-generation) antihistamines lies primarily in their ability to cross the blood-brain barrier (BBB). First-generation antihistamines, like diphenhydramine, are lipophilic and can easily cross the BBB, where they block histamine receptors in the brain, leading to drowsiness and sedation. Second-generation antihistamines, such as cetirizine and fexofenadine, are designed to be less lipophilic or to be actively pumped out of the brain, making them less able to cross the BBB. Consequently, they primarily act on peripheral histamine receptors, providing allergy relief without causing significant central nervous system side effects like sedation.

How do antacids work, and what are their common examples?

Antacids are basic substances that work by directly neutralizing the excess hydrochloric acid ($ ext{HCl}$) produced in the stomach. This neutralization reaction raises the pH of the stomach contents, providing relief from symptoms like heartburn, indigestion, and acid reflux. They do not prevent acid production, but rather counteract existing acid. Common examples include magnesium hydroxide ($ ext{Mg(OH)}_2$, often called Milk of Magnesia), aluminium hydroxide gel ($ ext{Al(OH)}_3$), and sodium bicarbonate ($ ext{NaHCO}_3$). While effective, overuse of sodium bicarbonate can lead to systemic alkalosis and rebound acid secretion.

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Antacids, Antihistamines

Drugs and their Classification

Chemistry

NEET UG

Version 1Updated 22 Mar 2026

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Drugs, in the context of chemistry and medicine, are chemical substances of low molecular mass (typically 100-500 amu) that interact with macromolecular targets within the body, such as proteins, nucleic acids, lipids, and carbohydrates, to produce a biological response. When these biological responses are therapeutic and desirable, these chemicals are called medicines. The systematic classificati…

Quick Summary

Drugs are chemical substances, typically of low molecular mass, that interact with biological macromolecules in the body to produce a biological response. When these responses are therapeutic, the drugs are termed medicines.

Drug classification is essential for understanding their action and use. Key classification methods include: by pharmacological effect (what they do, e.g., antacids, analgesics, antihistamines), by drug action or molecular target (how they work, e.

g., enzyme inhibitors, receptor agonists/antagonists), by chemical structure (common structural features, e.g., sulphonamides), and by molecular target (the specific macromolecule they bind to, e.g., proteins, nucleic acids).

Examples of drug classes include antacids (neutralize stomach acid), antihistamines (block histamine receptors), neurologically active drugs (affect CNS, e.g., tranquilizers), antimicrobials (kill/inhibit microbes, including antibiotics, antiseptics, disinfectants), and antifertility drugs (hormonal contraceptives).

Understanding these classifications and specific examples is crucial for NEET.

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Drugs: — Chemical substances interacting with macromolecular targets to produce biological response.

Medicines: — Drugs with therapeutic and desirable effects.

Classification by Pharmacological Effect:

* Antacids: Neutralize stomach acid (e.g., $\text{Mg(OH)}_2$ , $\text{Al(OH)}_3$ ). * Antihistamines: Block histamine receptors (e.g., Cetirizine, Fexofenadine). * Analgesics: Pain relievers (e.

g., Aspirin, Paracetamol). * Antipyretics: Reduce fever (e.g., Aspirin, Paracetamol). * Tranquilizers: Reduce anxiety (e.g., Diazepam, Barbiturates). * Antimicrobials: Kill/inhibit microbes.

* Antibiotics: Target bacteria (e.g., Penicillin - bactericidal; Tetracycline - bacteriostatic). * Antiseptics: For living tissues (e.g., Dettol, Tincture of Iodine). * Disinfectants: For inanimate objects (e.

g., 1% Phenol, Chlorine). * Antifertility Drugs: Prevent conception (e.g., Norethindrone, Ethynylestradiol).

Classification by Molecular Target: — Enzymes, Receptors (Agonists/Antagonists).

Agonist: — Mimics natural messenger, activates receptor.

Antagonist: — Blocks natural messenger, prevents receptor activation.

To remember key drug classes and their actions: All Animals Always Try And Make All Friends.