Cambridge IGCSE Biology 0610: Drugs – Revision Notes

These notes condense topic 15 (Drugs) of the Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028, section 15.1. Outcomes 15.1.1 to 15.1.4 are Core (Papers 1 and 3, and needed by Extended candidates too); outcome 15.1.5 is Supplement, labelled “Extended only” here and tested on Papers 2 and 4. For full explanations and worked examples, use the Drugs study guide.

Also useful:

• Drugs – Practice Questions with mark-by-mark answers
• Cambridge IGCSE Biology course hub and printable checklist
• Free 10-minute diagnostics: Core and Extended

The whole topic on one screen

Ref	Learn this	Tier
15.1.1	A drug is any substance taken into the body that modifies or affects chemical reactions in the body	Core
15.1.2	Antibiotics are used to treat bacterial infections	Core
15.1.3	Some bacteria are resistant to antibiotics, which reduces the effectiveness of antibiotics	Core
15.1.4	Antibiotics kill bacteria but do not affect viruses	Core
15.1.5	Using antibiotics only when essential limits the development of resistant bacteria such as MRSA	Extended only

That is the complete list. The topic in 0610 is about the definition of a drug and about antibiotics. Nothing else.

Key definitions

• Drug: any substance taken into the body that modifies or affects chemical reactions in the body. Learn it word for word.
• Antibiotic: a drug that kills bacteria (or stops them growing), used to treat bacterial infections.
• Bacterial infection: disease caused by pathogenic bacteria reproducing in the body.
• Resistant bacteria: bacteria that are not killed by an antibiotic.
• MRSA: methicillin-resistant Staphylococcus aureus, a strain of a common bacterium that is resistant to many antibiotics.
• Strain: a group of bacteria of one species that share a particular feature, such as resistance.

Antibiotics: what they do and do not do

	Bacteria	Viruses
Are they cells?	Yes	No
Have their own cell wall and chemical reactions?	Yes	No
Reproduce by…	dividing on their own	using the machinery inside a host cell
Affected by antibiotics?	Yes – killed (unless resistant)	No
Example infections	some throat and chest infections, infected wounds, cholera	colds, flu, HIV

Key sentence for a “Why not?” question: antibiotics act on structures and processes found in bacterial cells; viruses are not cells and do not have these, so antibiotics have nothing to act on.

Resistance – Core version

Two statements, both to be learned as sentences:

1. Some bacteria are resistant to antibiotics.
2. This reduces the effectiveness of antibiotics – an infection with resistant bacteria may not be cured, and a different antibiotic may be needed.

Resistance – Extended version (method box)

Explaining how resistance develops, in steps:

1. Variation: a random mutation makes a few bacteria resistant.
2. Selection: the antibiotic kills non-resistant bacteria.
3. Survival: resistant bacteria survive and have less competition.
4. Reproduction: resistant bacteria divide, passing on the resistance gene/allele.
5. Result: over many generations the proportion of resistant bacteria rises – a resistant strain such as MRSA.

Linking it to 15.1.5 (the step many answers miss):

6. Each use of an antibiotic is a chance to select resistant bacteria.
7. Using antibiotics only when essential (not for viral infections, not for minor infections the body can clear) means fewer chances for selection.
8. So resistant strains are less likely to develop and spread, and antibiotics stay effective.

This is natural selection. The same example is Supplement outcome 18.3.5 in topic 18, Variation and selection, so it can be examined there as well.

Must-know distinctions

• Resistant (bacteria) vs immune (people). Bacteria are never “immune” to an antibiotic.
• Kill bacteria vs affect viruses. Antibiotics do the first, not the second.
• Mutation is random vs selection is caused by the antibiotic. The antibiotic does not create resistance; it favours bacteria that already have it.
• Individuals do not change vs populations change. One bacterium does not “get used to” an antibiotic. The population changes because resistant ones survive and reproduce.
• Drug vs medicine. All medicines are drugs, but not all drugs are medicines. The definition says nothing about being helpful.
• Taken into the body vs made by the body. A hormone your body makes itself is not a drug under the 0610 definition.

Small worked reminders

Percentages from data: if 18 out of 150 samples are resistant, the percentage is (18 ÷ 150) × 100 = 12.0 %. Always convert to percentages before comparing years or hospitals with different sample sizes.

Growth by division: one bacterium dividing every 20 minutes for 3 hours goes through 180 ÷ 20 = 9 divisions, giving 2⁹ = 512 bacteria (if all survive). This is why a selected resistant strain spreads fast.

Wording a “Describe the trend” answer: give the direction, name the turning point, and quote figures with units – for example “rose from 6 % to 11 %, then fell to 8 %”.

Command words and what they want here

Command word	What a full answer on this topic looks like
State	A short fact with no reason: “antibiotics kill bacteria but do not affect viruses”.
Describe	Say what happens, in order: the antibiotic kills the bacteria, their number falls, the body clears the rest, the person recovers.
Explain	Give the reason as well as the fact: antibiotics do not help with flu because flu is caused by a virus, and viruses are not cells.
Suggest	Apply ideas to a new situation or to data, often with more than one acceptable answer: a fall in resistance after a policy change is consistent with less selection.
Calculate	Show the working, then give the answer to the precision asked, with the % sign.

Answer frames

Two-mark “Why won’t antibiotics cure this?” Point 1: the illness is caused by a virus. Point 2: antibiotics kill bacteria but do not affect viruses.

Four-mark “Explain how resistance developed” (Extended) Mutation gives resistance → antibiotic kills non-resistant bacteria → resistant bacteria survive and reproduce, passing on resistance → proportion of resistant bacteria increases (natural selection).

Four-mark “Explain how using antibiotics only when essential helps” (Extended) Mutation gives resistance → each use of the antibiotic selects resistant bacteria → using it only when essential means fewer chances for selection → resistant strains such as MRSA are less likely to develop and spread, so antibiotics stay effective.

Write each point as a full sentence. One idea per sentence makes it easy for an examiner to credit each marking point separately.

Quick self-test

Cover the answers and try each question first.

1. Give the full 0610 definition of a drug.
2. State what type of infection antibiotics are used to treat.
3. A patient has flu. Explain why a doctor does not prescribe an antibiotic.
4. State the effect of antibiotic resistance on the treatment of infections.
5. What does MRSA stand for?
6. (Extended) State where the resistance in a bacterial population first comes from.
7. (Extended) State what acts as the selection pressure when a resistant strain develops.
8. (Extended) Give two examples of using antibiotics only when essential.
9. Out of 150 samples, 18 were resistant. Calculate the percentage resistant.
10. A bacterium divides every 20 minutes. How many bacteria come from one cell after 3 hours, if all survive?
11. True or false: bacteria become immune to antibiotics.
12. Is a hormone made by your own ovaries a drug under the 0610 definition? Give a reason.

Answers

1. Any substance taken into the body that modifies or affects chemical reactions in the body.
2. Bacterial infections.
3. Flu is caused by a virus; antibiotics kill bacteria but do not affect viruses, so it would not help (and would give extra chances for resistant bacteria to be selected).
4. It reduces the effectiveness of antibiotics – the infection may not be cured and another antibiotic may be needed.
5. Methicillin-resistant Staphylococcus aureus.
6. A random mutation in a bacterium.
7. The antibiotic.
8. Any two: not using them for viral infections such as colds or flu; not using them for minor bacterial infections that the body can clear without help; using them only when a doctor judges that they are needed.
9. (18 ÷ 150) × 100 = 12.0 %.
10. 3 hours = 180 minutes; 180 ÷ 20 = 9 divisions; 2⁹ = 512.
11. False – bacteria become resistant; immunity refers to the body’s defences.
12. No – it is made by the body, not taken into the body.

Where marks are usually lost

• Writing “germs” or “microbes” instead of bacteria when saying what antibiotics kill.
• Saying antibiotics “do not work as well” on viruses. They do not affect viruses at all.
• Using “immune” for bacteria instead of resistant.
• Suggesting the antibiotic causes the mutation, or that bacteria “learn” or “adapt” to the drug.
• Describing natural selection in full on a 15.1.5 question but never saying why using antibiotics only when essential reduces selection.
• Leaving “taken into the body” out of the definition of a drug.
• Comparing raw numbers rather than percentages when sample sizes differ.
• Claiming data “prove” that less antibiotic use caused a fall in resistance. Use “suggests” or “is consistent with”.
• Forgetting that resistant bacteria pass on resistance when they reproduce – survival alone does not change the population.

Next steps

Try the Drugs practice questions, then move to topic 16 with the Reproduction revision notes.

Official syllabus

Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028 (Version 3), Cambridge Assessment International Education, part of Cambridge University Press & Assessment. Topic 15 Drugs, section 15.1.