Cambridge IGCSE Biology 0610: Drugs – Study Guide

This guide teaches topic 15 (Drugs) of the Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028. Topic 15 has one section, 15.1 Drugs, with four Core learning outcomes and one Supplement outcome. Core outcomes are tested on Papers 1 and 3 (and Extended candidates must know them too); the Supplement outcome, marked “Extended only” below, is tested on Papers 2 and 4. The topic is short, but it links directly to disease (topic 10) and natural selection (topic 18), so questions on it often carry more marks than its length suggests.

Useful pages for this topic:

• Course hub: Cambridge IGCSE Biology
• Printable checklist: Cambridge IGCSE Biology checklist
• Condensed notes: Drugs – Revision Notes
• Questions with worked answers: Drugs – Practice Questions
• Free 10-minute diagnostics: Core diagnostic and Extended diagnostic

What this topic covers

Syllabus ref	What you must be able to do	Tier
15.1.1	Describe a drug as any substance taken into the body that modifies or affects chemical reactions in the body	Core
15.1.2	Describe the use of antibiotics for the treatment of bacterial infections	Core
15.1.3	State that some bacteria are resistant to antibiotics, which reduces the effectiveness of antibiotics	Core
15.1.4	State that antibiotics kill bacteria but do not affect viruses	Core
15.1.5	Explain how using antibiotics only when essential can limit the development of resistant bacteria such as MRSA	Extended only

Notice what is not on the list. The 0610 syllabus does not ask you about named recreational drugs, alcohol, smoking or drug testing in this topic. Everything here is about the definition of a drug and about antibiotics.

15.1.1 What a drug is

The syllabus definition is precise, and you should learn it word for word:

A drug is any substance taken into the body that modifies or affects chemical reactions in the body.

Three parts of that definition earn credit:

1. “any substance” – a drug does not have to be a medicine. It can be helpful, harmful or neither.
2. “taken into the body” – it comes from outside. Substances your body makes for itself, such as hormones and enzymes, are not drugs under this definition.
3. “modifies or affects chemical reactions in the body” – this is the effect that makes it a drug.

A painkiller, an antibiotic and caffeine all fit the definition. A hormone such as insulin made by your own pancreas does not, because it was not taken into the body.

Worked example 1 – applying the definition

Question: A person drinks a cup of coffee. The caffeine in the coffee makes their heart beat faster. Explain why caffeine is described as a drug. [2]

Model answer:

• Caffeine is a substance taken into the body (it is swallowed in the drink). [1]
• It modifies or affects chemical reactions in the body, shown here by the change in heart rate. [1]

Both parts of the definition are needed for both marks. “It changes how your body works” on its own is too vague.

15.1.2 Using antibiotics to treat bacterial infections

A bacterial infection happens when pathogenic bacteria enter the body and reproduce. The body’s own defences (topic 10) often destroy them, but sometimes the infection spreads or becomes serious. A doctor may then prescribe an antibiotic.

What you need to describe:

• An antibiotic is a drug used to treat bacterial infections.
• It works by killing bacteria (or stopping them from growing and reproducing) inside the body.
• It does this by acting on structures or processes that bacterial cells have, but human cells do not, so the bacteria are harmed while body cells are not.
• As the number of bacteria falls, the body’s immune system can deal with the ones that remain, and the person recovers.

Examples of bacterial infections you may see in questions include some throat infections, some chest infections, infected wounds and cholera (which also appears in topic 10). Questions usually give you the context; you do not need a list of named antibiotics.

15.1.4 Why antibiotics do not affect viruses

This is a Core statement, and questions test it often: antibiotics kill bacteria but do not affect viruses.

The reason is that viruses are very different from bacteria:

• A bacterium is a living cell, with a cell wall, cell membrane, cytoplasm and its own chemical reactions. Antibiotics target these bacterial structures and processes.
• A virus is not a cell. It is a protein coat around genetic material, and it can only reproduce inside a host cell, using that cell’s machinery.
• So there is nothing in a virus for an antibiotic to act on.

This is why a doctor will not prescribe antibiotics for a cold or flu, which are caused by viruses. Taking an antibiotic for a viral infection gives no benefit to the patient. It also exposes the bacteria that live in and on the body to the antibiotic, which matters for resistance (see below).

15.1.3 Antibiotic resistance

The Core statement is short:

Some bacteria are resistant to antibiotics, which reduces the effectiveness of antibiotics.

A resistant bacterium is not killed by that antibiotic. If a person is infected with resistant bacteria, the antibiotic does not cure the infection, and the doctor has to try a different antibiotic. If the bacteria are resistant to several antibiotics, the infection may be very hard to treat.

For Core, that is all you need: state that resistance exists and that it makes antibiotics less effective.

15.1.5 How resistance develops and how to limit it (Extended only)

The Supplement outcome asks you to explain how using antibiotics only when essential limits the development of resistant bacteria such as MRSA. MRSA stands for methicillin-resistant Staphylococcus aureus, a strain of a common bacterium that many antibiotics no longer kill.

To explain it, you need the idea of natural selection, which you meet fully in topic 18 (Supplement outcome 18.3.5 uses antibiotic-resistant bacteria as its example).

How a resistant strain develops

1. In a large population of bacteria there is variation. A random mutation may give one bacterium resistance to an antibiotic.
2. When the antibiotic is used, it kills the non-resistant bacteria.
3. The resistant bacteria survive. They now have less competition.
4. The resistant bacteria reproduce (by dividing), passing on the gene for resistance to their offspring.
5. Over many generations, the proportion of resistant bacteria in the population increases. A resistant strain has developed.

The antibiotic did not cause the mutation. It acted as the selection pressure that let resistant bacteria out-survive the rest.

Why using antibiotics only when essential helps

Every time an antibiotic is used, non-resistant bacteria are killed and any resistant ones are favoured. So:

• The fewer times antibiotics are used, the fewer opportunities there are for resistant bacteria to be selected.
• Using them only when essential means not using them for viral infections (where they do nothing) and not using them for minor bacterial infections that the body can clear itself.
• With less selection, resistant bacteria are less likely to become common, and antibiotics stay effective for the patients who really need them.

Worked example 2 – an “Explain” answer (Extended)

Question: Explain how using antibiotics only when essential can limit the development of resistant bacteria such as MRSA. [4]

Model answer:

• Some bacteria in a population may have a mutation that makes them resistant. [1]
• When an antibiotic is used, non-resistant bacteria are killed but resistant bacteria survive and reproduce, passing on resistance. [1]
• This is natural selection, and the antibiotic is the selection pressure. [1]
• Using antibiotics only when essential gives fewer chances for resistant bacteria to be selected, so the resistant strain is less likely to spread. [1]

The last point is the one that answers the question. Many answers describe natural selection well and then stop.

Worked example 3 – reading resistance data

Question: A hospital tested samples of Staphylococcus aureus from patients over three years. Calculate the percentage of samples that were MRSA in each year and describe the trend. [4]

Year	Samples tested	Samples that were MRSA
1	250	20
2	240	42
3	200	22

Working:

Percentage = (number MRSA ÷ samples tested) × 100
Year 1: (20 ÷ 250) × 100 = 8.0 %
Year 2: (42 ÷ 240) × 100 = 17.5 %
Year 3: (22 ÷ 200) × 100 = 11.0 %

Trend: the percentage of MRSA rose from 8.0 % to 17.5 % between years 1 and 2, then fell to 11.0 % in year 3.

Note that you must compare percentages, not raw numbers, because the number of samples changed each year. If you were told the hospital cut unnecessary antibiotic use after year 2, you could suggest that the fall in year 3 is consistent with less selection for resistant bacteria. Say “suggest” or “consistent with” – the data alone do not prove the cause.

Worked example 4 – why resistance can spread quickly

Question: A single resistant bacterium divides once every 30 minutes. Assuming all offspring survive, how many resistant bacteria are there after 4 hours? [2]

Working:

4 hours = 240 minutes
Number of divisions = 240 ÷ 30 = 8
Number of bacteria = 2 to the power 8 = 256

Answer: 256 bacteria. This shows why a resistant strain, once selected, can become common very quickly.

How this topic links to others

• Topic 10, Diseases and immunity: pathogens, transmission and the body’s defences. Antibiotics help when those defences are not enough.
• Topic 18, Variation and selection (18.1): mutation is the way in which new alleles form, including the ones that give resistance.
• Topic 18, Variation and selection (18.3): antibiotic resistance is the syllabus’s named example of natural selection. An Extended question can ask it from either topic.

Common errors

• Writing “antibiotics kill viruses” or “antibiotics kill germs”. They kill bacteria only. “Germs” is too vague to credit.
• Saying bacteria become “immune” to antibiotics. Immunity is a feature of the human body (topic 10). Bacteria are resistant.
• Saying the antibiotic “causes” the mutation. Mutations are random and happen anyway. The antibiotic selects resistant bacteria that already exist.
• Saying individual bacteria “adapt” or “get used to” the antibiotic. The population changes because resistant bacteria survive and reproduce, not because individual bacteria change.
• Stopping at natural selection in a 15.1.5 answer. You must link back to using antibiotics only when essential.
• Leaving out “taken into the body” from the definition of a drug. Both halves of the definition are credited.
• Comparing raw numbers when sample sizes differ. Convert to percentages first.

Where to go next

Use the revision notes for a one-page summary and a quick self-test, then try the practice questions, which include data and extended-response questions with mark-by-mark answers. Topic 16 follows in the syllabus: start with the Reproduction study guide. Tick the topic off on the checklist when you can do every outcome in the table above without 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.