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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
A drug is any substance taken into the body that affects chemical reactions in it. Antibiotics kill bacteria but have no effect on viruses. Antibiotic resistance is natural selection in disguise: a few bacteria already carry a resistance mutation, the antibiotic kills the rest, and the survivors reproduce until resistance spreads.
How Cambridge tests this topic
Drugs is a recall topic with a few precise definitions that examiners mark strictly. Especially 'a drug is a substance taken into the body that modifies or affects chemical reactions'. The high-value question is antibiotic resistance, which is really natural selection in disguise, so the same six-mark chain applies. Learn the named components of tobacco smoke and what each one does, and this becomes quick, reliable marks.
Focused lessons
Break the topic into manageable skills
Use these guides when one concept, calculation or question pattern needs more attention than the full-topic overview.
Define a drug as any substance taken into the body that modifies or affects chemical reactions in the body. Antibiotics are drugs that kill bacteria (or stop them growing) and are used to treat bacterial diseases. The exam point students miss: antibiotics have no effect on viruses, because viruses reproduce inside the body's own cells and lack the bacterial structures antibiotics target. So a cold or flu (viral) cannot be treated with antibiotics. A recurring application question.
Antibiotic resistance: natural selection in a new costume
This is the topic's flagship six-marker, and it is the natural selection chain applied to bacteria:
In a population of bacteria there is variation; by chance, a few have a mutation giving resistance to the antibiotic.
When the antibiotic is used, it kills the non-resistant bacteria.
The resistant bacteria survive and reproduce.
They pass on the resistance allele to their offspring.
Over time the proportion of resistant bacteria increases, so the antibiotic becomes less effective.
Reducing the development of resistant strains is a standard add-on: only use antibiotics when necessary (not for viral infections), and always complete the full course. Note the precise causation. The antibiotic does not create resistance; it selects for bacteria that were already resistant. Saying 'the antibiotic makes the bacteria resistant' loses the mark.
Alcohol: effects to state precisely
Alcohol is a depressant that slows down reactions. Examined effects: it slows down reaction times (dangerous when driving), affects judgement and self-control, and long-term, excessive drinking damages the liver (cirrhosis. The liver is where alcohol is broken down). Questions often give a reaction-time experiment or a road-safety scenario; link the named effect to the consequence (slower reactions → longer stopping distance → more accidents).
Tobacco smoke: name the component, name the effect
Marks come from matching each named substance to its specific effect:
Carcinogen (causes lung cancer); damages cilia in the airways, so mucus and pathogens build up (smoker's cough, bronchitis)
Carbon monoxide
Combines with haemoglobin, reducing oxygen the blood can carry
The link to emphysema: damage to alveoli walls reduces surface area for gas exchange, so the smoker becomes breathless. As with alcohol, a vague 'smoking is bad for the lungs' scores nothing. Name the component and the precise effect.
Key terms to connect with this topic
Use these definitions inside explanations and questions rather than memorising them as isolated sentences.
Antibiotic resistance: The ability of a strain of bacteria to survive and reproduce in the presence of an antibiotic that would normally kill or inhibit it. Resistance arises through random mutations and spreads by natural selection: resistant bacteria survive, reproduce and pass on the resistance gene.
Cirrhosis: A disease of the liver caused by long-term excessive alcohol consumption, in which liver cells are damaged and replaced by scar tissue. Cirrhosis reduces the liver's ability to carry out its functions, including deamination, detoxification and bile production.
Drug dependence: The condition where a person's body has adapted to the presence of a drug and cannot function normally without it. If the drug is withdrawn, the person experiences unpleasant withdrawal symptoms such as headaches, nausea, anxiety or tremors.
Drug tolerance: The condition where a person needs increasing doses of a drug to achieve the same effect, because the body adapts to the presence of the drug. Tolerance develops over time with repeated use and can lead to increased consumption and greater health risks.
Withdrawal symptom: An unpleasant physical or psychological effect experienced when a person who is dependent on a drug suddenly stops taking it. Symptoms can include tremors, sweating, nausea, anxiety and pain, and vary depending on the drug.
Explain how a population of bacteria can become resistant to an antibiotic.
Show the answer and marking guidance
variation exists in the bacterial population / some bacteria have a mutation giving resistance; when the antibiotic is used it kills the non-resistant bacteria; resistant bacteria survive and reproduce; resistance allele passed on to offspring; proportion of resistant bacteria increases over generations / antibiotic becomes less effective;
Each semicolon separates a distinct answer point. This is original practice content, not an official Cambridge past-paper question.
Common mix-ups to avoid
Use these as a correction checklist after attempting a question.
Writing that the antibiotic 'makes' bacteria resistant. It selects for bacteria that were already resistant by mutation.
Giving vague harm ('smoking damages lungs') instead of naming the component and its specific effect.
Saying carbon monoxide 'poisons the lungs' rather than 'combines with haemoglobin, reducing oxygen transport'.
Forgetting that tar damages cilia, which is why mucus and pathogens accumulate.
Not completing the antibiotic course as a resistance-prevention point, or omitting prevention measures entirely.
How to build a complete longer answer
Antibiotic-resistance six-markers are scored exactly like a natural-selection answer, so reuse that scaffold: variation/mutation → selection pressure (the antibiotic) → survival of the resistant → reproduction → inheritance of the allele → increased frequency over generations. Crucially, phrase the cause correctly, 'bacteria that already had the resistance mutation survived', not 'the antibiotic caused resistance'. Finish with one or two prevention measures (complete the course; don't use antibiotics for viral infections) to mop up the application marks examiners attach to this question.
Over-prescription and over-the-counter access to antibiotics are live public-health issues in Malaysia, so resistance questions sometimes use local scenarios. Smoking statistics and vaping also feature; stick to the syllabus components (nicotine, tar, carbon monoxide) and their named effects rather than newer products the mark scheme doesn't cover.
Check your understanding
Practice this topic without notes
Attempt the questions before opening the marking guidance.
Antibiotics kill bacteria or stop them growing, but viruses reproduce inside the body's own cells and lack the bacterial structures antibiotics target. So colds and flu (viral) cannot be treated with antibiotics.
How does antibiotic resistance develop?
By natural selection: variation means a few bacteria carry a resistance mutation; the antibiotic kills the non-resistant bacteria; the resistant ones survive, reproduce and pass on the resistance allele, so over time the resistant strain becomes common. The antibiotic selects for resistance, it does not create it.
What are the effects of the components of tobacco smoke?
Nicotine is addictive and raises heart rate; tar is a carcinogen and damages the cilia in the airways; carbon monoxide combines with haemoglobin and reduces the oxygen the blood can carry. Name the component and its specific effect.