Drugs and Medicines

• How drugs interact with biological receptors to produce therapeutic effects
• Classification of analgesics (narcotic vs non-narcotic) with mechanism of action
• Key differences between antibiotics: bactericidal vs bacteriostatic, broad vs narrow spectrum
• Distinction between antiseptics and disinfectants with concentration-dependence examples
• Role of antacids, antihistamines, antifertility drugs, and tranquillisers

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Drug-Receptor Interaction

A drug is a chemical that interacts with a biological receptor (usually a protein) to produce a pharmacological response.

• Agonists — mimic natural substrate, activate receptor
• Antagonists — block receptor without activating it (most therapeutic drugs are antagonists)
• Competitive inhibition — drug competes with natural substrate for the same active site
• Non-competitive inhibition — drug binds elsewhere, changes shape of active site

Key idea: Drug efficacy depends on shape complementarity between drug molecule and receptor site (lock-and-key).

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Analgesics (Pain Relievers)

Aspirin (Acetylsalicylic acid): Irreversibly inhibits cyclooxygenase (COX) enzyme → reduces prostaglandin synthesis → reduces pain, fever, inflammation. Also used as antiplatelet agent (prevents clot formation).

Paracetamol: Inhibits COX centrally in CNS. Causes less GI irritation than aspirin. Overdose leads to hepatotoxicity (liver damage).

Morphine: Binds to μ-opioid receptors → decreased pain perception + euphoria. Highly addictive; classified as controlled substance.

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Antibiotics

Antibiotic = chemical produced by a microorganism (or synthetically) that inhibits growth of or kills other microorganisms.

Penicillin: Inhibits transpeptidase enzyme → prevents cross-linking of peptidoglycan in bacterial cell wall → bacteria lyse. Not effective against viruses.

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Antiseptics vs Disinfectants

Phenol is concentration-dependent: 0.2% → antiseptic; 1% → disinfectant.

Iodoform (CHI₃): Used as antiseptic for wounds; releases iodine slowly which kills bacteria.

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Antacids

Neutralise excess HCl in stomach to relieve hyperacidity.

Al(OH)₃ causes constipation; Mg(OH)₂ causes diarrhea. Combined in preparations like Gelusil/Maalox to balance side effects.

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Antihistamines

Histamine released in allergic reactions causes vasodilation, itching, and bronchoconstriction. Antihistamines competitively block histamine receptors.

• Cimetidine, Ranitidine — H₂ receptor antagonists; reduce gastric acid secretion; treat peptic ulcers
• Diphenhydramine, Chlorpheniramine — H₁ antagonists; treat allergic rhinitis, hay fever

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Antifertility Drugs

• Norethindrone — synthetic progestogen derivative; prevents ovulation by mimicking progesterone
• Combined oral contraceptive pills contain synthetic estrogen + progestogen

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Tranquillisers (Psychotherapeutic Drugs)

Serotonin is a neurotransmitter affecting mood. Low serotonin levels are associated with depression.

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Mechanism: Aspirin and COX Inhibition

Arachidonic acid ──[COX enzyme]──► Prostaglandins → pain, fever, inflammation

Aspirin: acetylates a serine –OH group in COX active site → IRREVERSIBLE inhibition
Ibuprofen: competitive, REVERSIBLE COX inhibitor

Aspirin's antiplatelet effect lasts the lifetime of a platelet (~8–10 days) because platelets lack nuclei and cannot synthesise new COX enzyme.

Penicillin: Structure and Resistance

• β-Lactam ring is the pharmacophore; structurally mimics the D-Ala-D-Ala terminus of peptidoglycan precursor
• Bacteria develop resistance by producing β-lactamase enzyme that hydrolyses the β-lactam ring → inactivating penicillin
• Amoxicillin + clavulanic acid (Augmentin): clavulanic acid irreversibly inhibits β-lactamase → restores penicillin activity

Drug Potency vs Efficacy

• Potency — dose required to produce a given effect (lower dose = higher potency)
• Efficacy — maximum therapeutic effect a drug can produce
• These are independent concepts: a drug can be highly potent with low efficacy

GABA and Benzodiazepines

Benzodiazepines (e.g., Valium) bind to GABA-A receptor at an allosteric site → increase frequency of Cl⁻ channel opening → hyperpolarisation of neuron → CNS depression → anti-anxiety and sedation.

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Example 1: Classify drugs by mechanism

Problem: Classify — Aspirin, Morphine, Valium, Penicillin, Ranitidine
by pharmacological class and mechanism.

Step 1: Analgesics
  Aspirin  → Non-narcotic analgesic; irreversibly inhibits COX enzyme
              → reduces prostaglandin synthesis
  Morphine → Narcotic analgesic; binds μ-opioid receptors in CNS
              → reduces pain perception + causes euphoria (addictive)

Step 2: Tranquilliser
  Valium (diazepam) → Benzodiazepine tranquilliser
                    → allosteric enhancement of GABA-A receptor
                    → increased Cl⁻ influx → CNS depression → anxiolytic

Step 3: Antibiotic
  Penicillin → Bactericidal antibiotic (narrow spectrum, gram +ve)
             → inhibits transpeptidase → prevents peptidoglycan
               cross-linking → cell wall rupture → bacterial death

Step 4: Antihistamine
  Ranitidine → H₂ receptor antagonist
             → reduces gastric acid secretion
             → used for peptic ulcer disease

Answer:
  Aspirin    — Non-narcotic analgesic (COX inhibitor)
  Morphine   — Narcotic analgesic (opioid receptor agonist)
  Valium     — Tranquilliser (GABA-A receptor modulator)
  Penicillin — Bactericidal antibiotic (cell wall synthesis inhibitor)
  Ranitidine — Antihistamine / H₂ blocker (reduces acid secretion)

Example 2: Antiseptic vs Disinfectant

Problem: Explain why phenol at 0.2% is used on wounds but at 1% only on
surfaces. What general rule does this illustrate?

Step 1: Phenol at 0.2%
  → Low concentration; does not denature human tissue proteins significantly
  → Still lethal to bacteria on wound surface
  → Classified as ANTISEPTIC (safe for living tissue)

Step 2: Phenol at 1%
  → Higher concentration causes rapid protein denaturation
  → Toxic to mammalian cells → NOT safe for skin/wound use
  → Suitable for disinfecting non-living surfaces (floors, instruments)
  → Classified as DISINFECTANT

Step 3: General Rule
  The SAME chemical can act as antiseptic OR disinfectant
  depending on concentration.
  
  Lower concentration → antiseptic (living tissue)
  Higher concentration → disinfectant (non-living surfaces)

  This also applies to H₂O₂:
    3% → antiseptic (wound cleaning)
    >10% → industrial bleach/disinfectant

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Q1. Which of the following is a narcotic analgesic?

A) Aspirin
B) Paracetamol
C) Morphine
D) Ibuprofen

Q2. Penicillin acts as a bactericidal antibiotic by:

A) Inhibiting protein synthesis in ribosomes
B) Disrupting bacterial plasma membrane
C) Inhibiting peptidoglycan cell wall synthesis
D) Blocking bacterial RNA polymerase

Q3. Ranitidine is best classified as:

A) An antacid that neutralises HCl
B) A broad-spectrum antibiotic
C) An H₂ receptor antagonist
D) An antifertility drug

Q4. A drug that blocks a receptor without activating it is called:

A) Agonist
B) Competitive agonist
C) Antagonist
D) Allosteric activator

Q5. Iodoform (CHI₃) acts as an antiseptic because it:

A) Directly denatures bacterial proteins on contact
B) Releases iodine slowly which kills bacteria
C) Raises pH of wound environment
D) Inhibits bacterial cell wall synthesis

Answer Key: 1-C | 2-C | 3-C | 4-C | 5-B

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• NCERT Class 12 Chemistry — Chapter 16: Chemistry in Everyday Life
• Pages 474–486 (drugs and medicines)
• Key sections: 16.1 Drug-target interaction, 16.2 Therapeutic action of different classes of drugs
• Table 16.1: Classification of drugs by pharmacological effect
• Refer also to NCERT Exemplar Problems, Chapter 16 for additional MCQs

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🎯 Exam Tips

• JEE frequently asks "Which is bactericidal/bacteriostatic?" — memorise the short list cold
• Drug questions often use trade names (Valium, Luminal, Prozac) — know both trade and generic names
• Antiseptic vs disinfectant distinction appears almost every year — anchor on concentration and tissue safety
• Serotonin as neurotransmitter for depression/antidepressants is a reliable one-liner MCQ
• Aspirin's IRREVERSIBLE COX inhibition vs ibuprofen's REVERSIBLE inhibition is a JEE distinction point

📊 Weightage

• 1–2 questions per JEE Main paper from Chemistry in Everyday Life
• Chapter contributes ~4 marks to JEE Main annually
• Recall-based chapter — high return for time invested in revision

🔗 Related Topics

• Biomolecules (enzyme-substrate analogy parallels drug-receptor interaction)
• Amines (most drugs contain amine functional groups)
• Alcohols and Phenols (antiseptic chemistry of phenol, iodoform)
• Carbonyl Compounds (aspirin synthesis from salicylic acid)

📝 Revision Checklist

• Name 2 narcotic and 3 non-narcotic analgesics with mechanism
• Distinguish bactericidal vs bacteriostatic with 2 examples each
• List antiseptics with active ingredients (Dettol, Bithional, Iodoform)
• Write neutralisation reactions of NaHCO₃, Mg(OH)₂, Al(OH)₃ with HCl
• Recall H₁ vs H₂ antihistamines with clinical use
• State mechanism of penicillin and basis of resistance
• Identify norethindrone's class and action
• Name 3 tranquilliser classes with one example each