Drugs
This is the shortest topic on the syllabus. It has one definition, one class of medicine, and one important process — antibiotic resistance — which links directly to natural selection.
1. What a drug is
A drug is any substance taken into the body that modifies or affects chemical reactions in the body.
Note how broad that is. It covers medicines prescribed by a doctor as well as substances taken for other reasons — the definition is about the effect on the body’s chemistry, not about whether the substance is helpful or harmful.
2. Antibiotics
Antibiotics are drugs used to treat bacterial infections.
They work by killing bacteria or stopping them from multiplying, without damaging human cells. Penicillin — produced by the fungus Penicillium — is the best-known example.
Antibiotics do not work against viruses
This is the single most examined point in the topic, and the reasoning matters as much as the fact.
Antibiotics work by attacking structures and processes found in bacterial cells — the cell wall, for example, or bacterial protein synthesis. Viruses have none of these. A virus is only genetic material in a protein coat, and it reproduces inside the host’s own cells, so there is no bacterial machinery for the antibiotic to act on.
Taking antibiotics for a cold or flu is useless — both are caused by viruses. Worse, it is actively harmful at population level, because it contributes to resistance without treating anything.
3. Antibiotic resistance
Some bacteria are resistant to antibiotics — the antibiotic no longer kills them. MRSA is the widely cited example.
How resistance develops — this is natural selection, and should be written as a sequence:
- Within a bacterial population there is variation, caused by random mutation
- A mutation produces an allele giving resistance to the antibiotic
- When the antibiotic is used, the non-resistant bacteria are killed
- The resistant bacteria survive, and now have less competition
- They reproduce rapidly, passing on the resistance allele
- Over time the proportion of resistant bacteria increases in the population
The mutation comes first. Bacteria do not become resistant because they were exposed to an antibiotic. The resistant individuals were already present by chance; the antibiotic simply selected for them by killing everything else. Answers that reverse this describe the wrong mechanism and lose the marks.
Reducing the development of resistance
- Only use antibiotics when they are genuinely necessary — not for viral infections
- Always complete the full course, even after symptoms improve, so that no partially resistant bacteria survive to reproduce
- Reduce routine use of antibiotics in livestock farming
- Good hospital hygiene to limit the spread of resistant strains
“Finish the course” has a mechanism behind it. Stopping early leaves behind the bacteria that were hardest to kill — precisely the ones most likely to carry resistance alleles — and gives them a population with no competition to multiply into.
4. Mistakes that cost marks
Saying antibiotics kill viruses. They act only on bacteria.
Saying bacteria “become resistant because antibiotics were used.” Resistance arises by random mutation first; the antibiotic then selects for it.
Saying the bacteria “learn to resist” or “get used to” the antibiotic.
Saying resistance is passed on “by the antibiotic.” It is passed on genetically, when resistant bacteria reproduce.
Defining a drug as “a medicine.” The definition is any substance taken into the body that modifies chemical reactions in it.
Giving only “finish the course” without the reason.
Frequently asked questions
What is a drug? Any substance taken into the body that modifies or affects chemical reactions in the body.
What do antibiotics do? They treat bacterial infections, by killing bacteria or preventing them from multiplying.
Why don’t antibiotics work on viruses? Antibiotics target structures and processes found in bacterial cells, such as the cell wall. Viruses do not have these, and reproduce inside the host’s own cells.
How does antibiotic resistance develop? A random mutation produces a resistance allele. The antibiotic kills the non-resistant bacteria, the resistant ones survive and reproduce, and their proportion increases — natural selection.
Why should you finish a course of antibiotics? So that all bacteria are killed. Stopping early leaves the hardest-to-kill bacteria alive to reproduce.
What is MRSA? A strain of bacteria resistant to several antibiotics, commonly used as the example of antibiotic resistance.
Quick revision checklist
- I can define a drug precisely
- I know what antibiotics treat and how they work
- I can explain why antibiotics do not affect viruses
- I can write antibiotic resistance as a full natural selection sequence
- I know the mutation comes before the antibiotic is used
- I can give ways to reduce the development of resistance, with reasons
- I can explain why finishing a course of antibiotics matters
These notes cover topic 15 of the Cambridge IGCSE Biology (0610) syllabus and are written for Grade 9–11 / Year 10–11 students. This was the least-covered topic in the corpus of lessons behind this project — around a tenth of the material available for the largest topics — so the page follows the syllabus closely and is deliberately concise rather than padded. Older versions of this syllabus, and some other specifications, also cover misused drugs such as alcohol, nicotine and anabolic steroids; check your own syllabus document before revising those.
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