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Learn5090Paper 2 · TheoryDisease and immunityMedicinal and misused drugs
Disease and immunity · Subtopic 2 of 2

Drugs

A drug is any substance taken into the body that changes its chemical reactions. Antibiotics kill or stop the growth of bacteria but have no effect on viruses. Overuse leads to antibiotic resistance: random mutations make some bacteria resistant; they survive and multiply (natural selection), e.g. MRSA. Reduce it by prescribing antibiotics only when needed and finishing the course. Misused drugs harm the body: alcohol (depressant, liver damage), heroin (depressant, highly addictive, infection from shared needles), nicotine (addictive), anabolic steroids (muscle growth, banned in sport).

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3≈ 30 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Define a drug.
  • Explain how antibiotics are used and why they don't work on viruses.
  • Explain how antibiotic resistance develops and how to reduce it.
  • Describe the effects of misused drugs.
Pause and think

Why do doctors tell you to finish a course of antibiotics even when you feel better?

Section 2 of 14

Core idea

Start hereA miracle at risk

Before antibiotics, a scratch could be fatal. Now some bacteria resist almost every antibiotic we have, and doctors warn of a return to that world.

The big idea

Drugs change body chemistry. Antibiotics kill bacteria but overuse selects for resistant strains. Misused drugs can cause addiction and organ damage.

1. Antibiotic resistance

How resistance spreads

Natural selection in action.

Worked exampleAdvice to a patient

A patient with a cold asks for antibiotics.

Goal
Explain the doctor's refusal.
Strategy
Cause of disease and resistance risk.
  1. cold=virus\text{cold}=\text{virus}
    Antibiotics don't affect viruses.
  2. unnecessary use\text{unnecessary use}
    Exposes bacteria to antibiotics, selecting resistant strains.
AnswerNo benefit and increases resistance.
✓ CheckRest and fluids are the advice ✓.
Key takeawayMisused drugs Alcohol and heroin are depressants; heroin is highly addictive. Nicotine is addictive. Anabolic steroids build muscle and are banned in sport.
Pause and think

Why are hospitals hotspots for resistant bacteria like MRSA?

Summary

  • Antibiotics: bacteria only.
  • Resistance by natural selection.
  • Misused drugs harm health.
Key words
Drug
Substance that changes chemical reactions in the body.
Antibiotic
Drug that kills or stops growth of bacteria.
Resistance
Ability of bacteria to survive an antibiotic.
HistoryFleming's warning. In his 1945 Nobel lecture, Alexander Fleming warned that misusing penicillin could make bacteria resistant. Within decades, he was proved right.
Fun factAncient antibiotics. Bacteria and fungi have been making antibiotics to fight each other for millions of years; resistance genes have even been found in 30,000-year-old permafrost.
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