Common Drugs: Antacids, Antihistamines, Antibiotics & More

Drugs in everyday life fall into a few clear families, each with a different chemical job. Antacids neutralise excess stomach acid, antihistamines block the histamine behind allergies, tranquilizers calm the central nervous system, analgesics relieve pain, antibiotics kill bacteria, and antiseptics disinfect living tissue — and confusing one family with another is a classic exam trap.

Knowing these categories matters beyond exams: it explains why an antibiotic cannot cure a cold (colds are viral), why antacids and antihistamines are not interchangeable despite the similar names, and how a tiny structural difference turns a medicine into a poison at the wrong dose.

Key Concepts

• Antacids: bases that neutralise excess HCl in the stomach — e.g. magnesium hydroxide (milk of magnesia), aluminium hydroxide, sodium bicarbonate.
• Antihistamines: block H1 histamine receptors to relieve allergy symptoms — e.g. cetirizine, diphenhydramine; they treat allergy, not acidity.
• Tranquilizers: CNS depressants for anxiety and tension — benzodiazepines (diazepam/Valium) and barbiturates; can be habit-forming.
• Analgesics: pain relievers — aspirin (also antipyretic and anti-inflammatory, an NSAID) and paracetamol (antipyretic, gentler on the stomach).
• Antibiotics: kill or inhibit bacteria — penicillin blocks bacterial cell-wall synthesis; broad-spectrum vs narrow-spectrum; useless against viruses.
• Antiseptics vs disinfectants: antiseptics are applied to living tissue (Dettol = chloroxylenol + terpineol; tincture of iodine), disinfectants to inanimate objects.

Modern acid control also uses proton-pump inhibitors like omeprazole, which reduce acid production rather than neutralising acid already made. Aspirin works by inhibiting prostaglandin synthesis, while paracetamol acts mainly in the brain's pain and temperature centres.

Formulas

• Antacid neutralisation: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O; NaHCO₃ + HCl → NaCl + H₂O + CO₂. Each mole of Mg(OH)₂ neutralises 2 moles of HCl; each mole of NaHCO₃ neutralises 1 mole.
• Dose arithmetic: total drug = (mass per tablet) × (number of tablets); compare against the maximum daily dose — e.g. paracetamol's commonly cited adult limit is 4 g per day.
• Assumption: the neutralisation goes to completion in the stomach's acidic medium; tablet masses are taken as pure active ingredient for the calculation.

Worked Example

An antacid tablet contains 250 mg of sodium bicarbonate (NaHCO₃, M = 84 g mol⁻¹). How much stomach HCl can one tablet neutralise? Step 1: moles of NaHCO₃ = 0.250 ÷ 84 = 0.00298 mol. Step 2: the equation NaHCO₃ + HCl → NaCl + H₂O + CO₂ shows a 1:1 mole ratio. Step 3: moles of HCl neutralised = 0.00298 mol. Step 4: mass of HCl = 0.00298 × 36.5 = 0.109 g. So one tablet neutralises about 0.11 g of hydrochloric acid.

Common Misconceptions

• "Antibiotics cure colds and flu." False — those are viral infections; antibiotics target bacteria, and misuse breeds resistant superbugs.
• "Antacids and antihistamines do the same thing." No — antacids neutralise stomach acid while antihistamines block histamine receptors for allergies; the names just sound alike.
• "If a drug is sold over the counter, any dose is safe." Wrong — paracetamol overdose causes severe liver damage; dose limits exist for every drug.

Real-World Applications

• Everyday treatment: antacids for acidity, cetirizine for dust allergy, paracetamol for fever — the three most common self-medication choices.
• Surgery and hospitals: antiseptics (povidone-iodine, chlorhexidine) sterilise skin before operations; antibiotics prevent post-surgical infection.
• Public health: antibiotic stewardship programmes fight resistance; vaccines plus antibiotics together have doubled human life expectancy in a century.

Each drug family has one chemical mission — neutralise acid, block histamine, calm the CNS, relieve pain, kill bacteria or disinfect tissue. Learn one mechanism and one example per family, and never mix up an antacid with an antihistamine.