Cambridge IGCSE Biology 0610 · Topic 15.1
Drugs
Chapter 15 is deliberately narrow — one syllabus point, three ideas, no wasted material. You need to know what actually counts as a drug in Biology, why antibiotics work against bacteria but do nothing at all to a virus, and why some bacteria stop responding to antibiotics that used to kill them. Get those three ideas straight, in that order, and this chapter is done.
Cambridge groups all of it under a single syllabus point, 15.1 Drugs, and it rewards the same discipline it rewards everywhere else in Biology: precise wording over a roughly-right general idea. “Antibiotics fight infection” sounds reasonable and earns nothing, because it doesn’t say which kind of infection. “Bacteria get immune to medicine” sounds reasonable and is actually wrong, because resistance belongs to the bacteria, not to the person taking it. The pages below exist to close exactly those two gaps.
What is a drug?
Start with the definition, because everything else in the chapter depends on getting it exactly right. A drugis any substance taken into the body that modifies or affects chemical reactions in the body. Two parts have to both be true before something counts: it has to be taken into the body, and it has to affect a chemical reaction going on inside it. Drop either half and the definition falls apart — “something that changes the body” is too loose, because it never mentions chemistry at all.
Notice what the definition does notsay. It doesn’t mention legality, and it doesn’t mention harm. A substance doesn’t need to be illegal, addictive or dangerous to count as a drug in Biology — it just needs to enter the body and affect its chemistry. That makes the category wider than most students expect: antibiotics, insulin and ordinary pain-relief medicines such as paracetamol all fit it, sitting in exactly the same biological category even though they have nothing in common legally or socially.
It also means the definition has a boundary worth knowing from the other side. Drinking water to relieve thirst doesn’t count as taking a drug, and neither does receiving a blood transfusion, even though in both cases something is clearly entering the body. Neither one is defined by a chemical-reaction effect in the way the syllabus means it, so the test to apply is always the definition itself — taken into the body, and affecting a chemical reaction — never just “did something go in?”
The rest of this chapter is about one particular type of drug: antibiotics, what they can treat, and what happens when bacteria stop responding to them.
What’s in this chapter
| Page | Level | What it covers |
|---|---|---|
| Antibiotics and bacterial infections | Core | Why antibiotics treat bacterial infections but have no effect at all on a virus, and the biological reason the two cases are different. |
| Antibiotic resistance and MRSA | Core + Supplement | How resistant bacteria survive antibiotic exposure and become more common, why essential-only use slows this down, and what MRSA actually demonstrates. |
Core or Supplement?
The definition of a drug and the page on antibiotics and bacterial infections are both Core, so every candidate needs them regardless of tier. Antibiotic resistance is split: the fact that resistant bacteria exist and reduce an antibiotic’s effectiveness is Core, but explaining whyusing antibiotics only when essential can limit the development of resistant bacteria such as MRSA is Supplement only. If you’re sitting Core, you can stop partway through that page — it tells you exactly where.