ChemistryCBSE Class 12 · Chemistry in Everyday Life

Drug-Target Interaction and Classes of Drugs: Class 12 Chemistry

Updated 18 October 2026 · Fact-checked

Drugs are chemicals of low molecular mass (about 100 to 500 u) that interact with macromolecular targets such as enzymes and receptors to produce a biological response. Competitive and allosteric inhibitors block enzymes. Antacids neutralise or reduce stomach acid, antihistamines block histamine, tranquilisers act on nerve signalling, and analgesics relieve pain without loss of consciousness.

Understand Drug-Target Interaction, Antacids, Antihistamines, Tranquilisers and Analgesics

Drugs and targets. Drugs are chemicals of low molecular mass (about 100 to 500 u) that interact with large molecules in the body to produce a biological response. When the response is therapeutic and useful they are medicines, used to diagnose, prevent and treat disease; most drugs are potential poisons at higher than recommended doses. Using chemicals for therapeutic effect is chemotherapy. Targets include enzymes, receptors, carrier proteins, nucleic acids, lipids and carbohydrates.

Enzymes as targets. An enzyme holds its substrate in the active site through ionic bonds, hydrogen bonds, van der Waals or dipole-dipole interactions, and provides functional groups that attack the substrate. Enzyme inhibitors block these actions. Competitive inhibitors compete with the natural substrate for the active site. Some drugs bind to a different site, the allosteric site, changing the shape of the active site so the substrate cannot recognise it. If the enzyme-inhibitor bond is strong and covalent, the enzyme is blocked permanently; the body breaks down the complex and makes new enzyme.

Antacids. Excess stomach acid causes irritation, pain and, in severe cases, ulcers. Early treatment used sodium hydrogencarbonate or a mixture of aluminium and magnesium hydroxides. Too much hydrogencarbonate can make the stomach alkaline and trigger more acid; insoluble metal hydroxides do not raise the pH above neutral. They treat only symptoms. Histamine stimulates secretion of pepsin and HCl, so cimetidine (Tagamet) was designed to stop histamine from interacting with receptors in the stomach wall; ranitidine (Zantac) followed.

Antihistamines. Histamine is a potent vasodilator; it contracts smooth muscles in bronchi and gut, relaxes muscles in fine blood vessels and causes nasal congestion and the allergic response to pollen. Brompheniramine (Dimetapp) and terfenadine (Seldane) compete with histamine for receptor sites. They do not reduce stomach acid because anti-allergic and antacid drugs act on different receptors.

Tranquilisers. They treat stress and mental illness by relieving anxiety, stress, irritability or excitement. Low noradrenaline causes depression; iproniazid and phenelzine inhibit the enzymes that break noradrenaline down. Chlordiazepoxide and meprobamate are mild tranquilisers; equanil controls depression and hypertension. Barbiturates (veronal, amytal, nembutal, luminal, seconal) are hypnotics.

Analgesics. They reduce pain without impairing consciousness. Non-narcotic: aspirin and paracetamol. Aspirin inhibits synthesis of prostaglandins, which cause inflammation and pain; it relieves skeletal pain, reduces fever and prevents platelet coagulation, so it helps prevent heart attacks. Narcotic: morphine and homologues (opiates, from the opium poppy); in medicinal doses they relieve pain and produce sleep, in poisonous doses they cause stupor, coma, convulsions and death.

Drug-Target Interaction, Antacids, Antihistamines, Tranquilisers and Analgesics: formulas and key facts

Enzyme inhibition: Competitive blocks active site
Allosteric binds elsewhere, changes active-site shape
Covalent binding permanent block; Body makes new enzyme later.
Drug classes: $A n t a c i d : c im e t i d in e, r ani t i d in e ∣ A n t ihi s t amin e : b r o m p h e ni r amin e, t er f e na d in e ∣ T r an q u i l i ser : i p r o nia z i d, p h e n e l z in e, e q u ani l ∣ A na l g es i c : a s p i r in, p a r a ce t am o l, m or p hin e$; Learn each with its class.
Analgesic types: $N o n - na r co t i c : a s p i r in, p a r a ce t am o l ∣ N a r co t i c : m or p hin e, o p ia t es$; Aspirin inhibits prostaglandin synthesis.

Competitive	blocks active site
Allosteric	binds elsewhere, changes active-site shape
Covalent binding	permanent block

How to solve Drug-Target Interaction, Antacids, Antihistamines, Tranquilisers and Analgesics questions

Formative (school) assessment only in 2026-27. Questions ask for classification, mode of action and reasoning about doses.

1Name the target: enzyme or receptor.
2Say how the drug acts: competes, binds an allosteric site or blocks a receptor.
3For a class, give two examples.
4For differences, use fixed heads such as source, addiction and action.
5For dilution or dose arithmetic, use C1V1 = C2V2 and mg per kg.

Drug-Target Interaction, Antacids, Antihistamines, Tranquilisers and Analgesics in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Why not just an antihistamine for acidity

Suppose a student with acidity takes an anti-allergy tablet and finds no relief.

Explain why.

Try it first, then show the solutionSolution

Antihistamines act on receptors for allergy.
Antacid drugs like cimetidine act on different receptors in the stomach wall.

Answer: Antihistamines such as brompheniramine act on different receptors from those controlling stomach acid, which cimetidine targets, so an antihistamine does not reduce acidity.

Memory hook: Different receptors, different drugs.

Dose and poison

Suppose a label warns that a painkiller must not exceed the stated daily dose.

Why can a useful drug be harmful above that dose?

Try it first, then show the solutionSolution

Most drugs are potential poisons at higher than recommended doses.

Answer: Most drugs are potential poisons at doses higher than recommended, and morphine, for example, produces stupor, coma and convulsions at poisonous doses, so the dose limit matters.

Memory hook: Dose makes the poison.

Tricks to remember Drug-Target Interaction, Antacids, Antihistamines, Tranquilisers and Analgesics

Short memory aids. Say each one aloud once, then use it on a question the same day.

Cimetidine blocks histamine in the stomach
Cim-etidine: stops acid release at stomach receptors.
Use it when: Antacid design.
Aspirin stops prostaglandins
A for aspirin, anti-prostaglandin.
Use it when: Mode of action.
Opium = narcotic
Morphine from the opium poppy is narcotic.
Use it when: Analgesic types.

Drug-Target Interaction, Antacids, Antihistamines, Tranquilisers and Analgesics: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Non-narcotic analgesic · 1 mark · MCQBoard level

Which of the following is a non-narcotic analgesic?
(a) morphine
(b) aspirin
(c) cimetidine
(d) terfenadine

Try it first, then show the solutionSolution

Morphine is narcotic, cimetidine an antacid, terfenadine an antihistamine; aspirin is non-narcotic (1 mark).

Answer: (b) aspirin

Example 2: Competitive vs allosteric (2 marks) · 2 marks · Very short answerBoard level

Distinguish competitive from allosteric enzyme inhibitors. (2 marks)

Try it first, then show the solutionSolution

Competitive inhibitors compete with the substrate for the active site (1 mark).
Allosteric inhibitors bind a different site and change the shape of the active site so the substrate cannot recognise it (1 mark).

Answer: A competitive inhibitor competes with the substrate for the active site. An allosteric inhibitor binds elsewhere and changes the shape of the active site so the substrate cannot bind.

Example 3: Why cimetidine was designed (4 marks) · 4 marks · Short answerHarder board level

Explain how the discovery of the role of histamine led to cimetidine, and why antacids such as metal hydroxides are preferred to sodium hydrogencarbonate. (4 marks)

Try it first, then show the solutionSolution

Histamine stimulates secretion of pepsin and HCl (1 mark).
Cimetidine stops histamine interacting with receptors in the stomach wall, so less acid is released (1 mark).
Excess hydrogencarbonate can make the stomach alkaline and trigger more acid (1 mark).
Insoluble metal hydroxides do not raise the pH above neutral (1 mark).

Answer: Histamine stimulates secretion of pepsin and HCl. Cimetidine was designed to stop histamine from interacting with receptors in the stomach wall, so less acid is released. Excess sodium hydrogencarbonate can make the stomach alkaline and trigger more acid, whereas insoluble metal hydroxides do not raise the pH above neutral.

Example 4: Analgesics compared (5 marks) · 5 marks · Long answerHarder board level

Compare narcotic and non-narcotic analgesics with two examples, mode of action and two uses of each. (5 marks)

Try it first, then show the solutionSolution

Non-narcotic: aspirin and paracetamol (1 mark).
Aspirin inhibits prostaglandin synthesis, so reduces inflammation and pain (1 mark).
Uses: skeletal pain, fever, preventing platelet coagulation (1 mark).
Narcotic: morphine and its homologues from the opium poppy (1 mark).
Uses: postoperative pain, cardiac pain, terminal cancer pain, childbirth; poisonous doses cause stupor, coma, convulsions, death (1 mark).

Answer: Non-narcotic analgesics, such as aspirin and paracetamol, are non-addictive. Aspirin inhibits prostaglandin synthesis, relieves skeletal pain, reduces fever and prevents platelet coagulation. Narcotic analgesics, such as morphine and other opiates from the opium poppy, relieve postoperative, cardiac and terminal-cancer pain and pain in childbirth, but poisonous doses cause stupor, coma, convulsions and death.

Example 5: Antidepressants reasoning · 4 marks · Case-basedHOTS / competency

Iproniazid is used in depression. (i) What neurotransmitter is low in depression? (1) (ii) How does iproniazid act? (2) (iii) Name one mild tranquiliser. (1)

Try it first, then show the solutionSolution

(i) Noradrenaline (1 mark).
(ii) It inhibits the enzymes that break down noradrenaline, so noradrenaline acts on its receptor for longer (2 marks).
(iii) Chlordiazepoxide or meprobamate (1 mark).

Answer: (i) Noradrenaline. (ii) Iproniazid inhibits the enzymes that break down noradrenaline, so it stays active longer at its receptor. (iii) Chlordiazepoxide (or meprobamate).

Where students lose marks in Drug-Target Interaction, Antacids, Antihistamines, Tranquilisers and Analgesics

The slips examiners see most often, and what to write instead.

Mistake: Calling aspirin narcotic
Pain relievers lumped together.
Wrong working
Aspirin is a narcotic analgesic.
Right working
Aspirin is non-narcotic; morphine is narcotic.
Do this: Opium poppy means narcotic.
Usually costs: 1 mark.
Mistake: Antacid means a base only
Ignoring receptor-blocking drugs.
Wrong working
Cimetidine neutralises acid.
Right working
Cimetidine stops histamine acting on stomach receptors, reducing acid release.
Do this: Say 'blocks receptors'.
Usually costs: 1 mark.
Mistake: Confusing inhibitor types
Both stop the enzyme.
Wrong working
Allosteric inhibitors bind the active site.
Right working
Allosteric inhibitors bind a different site and change the active site's shape.
Do this: Allo = other.
Usually costs: 1 mark.

Exam tips

Chemistry in Everyday Life is formative (school) assessment only in 2026-27.
Give two examples per drug class.
Link every drug to a target: enzyme or receptor.

Practice questions on Drug-Target Interaction, Antacids, Antihistamines, Tranquilisers and Analgesics

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Terfenadine acts as:
(a) an antacid
(b) an antihistamine
(c) a narcotic analgesic
(d) a barbiturate
Hint
Competes with histamine.
Answer
(b) an antihistamine
Q2 · 2 marks · Very short answerBoard level
What is chemotherapy? (2 marks)
Hint
Chemicals for therapeutic effect.
Answer
The use of chemicals for therapeutic effect, to diagnose, prevent or treat diseases.
Q3 · 3 marks · Short answerHarder board level
A drug binds covalently to an enzyme's active site. Explain what happens to the enzyme and how the body recovers. (3 marks)
Hint
Permanent blocking.
Answer
A strong covalent bond blocks the enzyme permanently. The body breaks down the enzyme-inhibitor complex and synthesises new enzyme.
Q4 · 5 marks · Long answerHarder board level
Explain why antihistamines do not reduce stomach acid although both histamine-related drug types compete with histamine, and describe how cimetidine works. (5 marks)
Hint
Different receptors.
Answer
Histamine acts on different receptors in different tissues. Antihistamines such as brompheniramine and terfenadine compete with histamine at the receptors involved in allergy and nasal congestion, not those in the stomach wall. Cimetidine was designed to prevent histamine from interacting with the stomach-wall receptors, so less pepsin and HCl are secreted. Because the receptors differ, anti-allergic and antacid drugs are not interchangeable.
Q5 · 1 mark · Assertion-reasonHOTS / competency
Assertion (A): Morphine, taken in a poisonous dose, can cause stupor, coma and death.
Reason (R): Morphine is a non-narcotic analgesic like paracetamol.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Hint
Is morphine narcotic?
Answer
(c) A is true, but R is false. Morphine is a narcotic analgesic.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What are the targets of drugs?Show answer
Enzymes, receptors, carrier proteins, nucleic acids, lipids and carbohydrates.
2.What does aspirin inhibit?Show answer
The synthesis of prostaglandins.
3.Name two barbiturates.Show answer
Any two of veronal, amytal, nembutal, luminal, seconal.
4.Which drug was the world's largest selling before ranitidine?Show answer
Cimetidine (Tagamet).

Drug-Target Interaction, Antacids, Antihistamines, Tranquilisers and Analgesics: frequently asked questions

What is a drug?

A low molecular mass chemical that interacts with macromolecular targets to produce a biological response.

What are analgesics?

Drugs that reduce or abolish pain without impairing consciousness.

What does an antihistamine do?

It competes with histamine for its receptor binding sites.