Introduction
Okay, real talk: this is one of the shortest chapters in the whole course. There's barely anything to learn, which is exactly why the exam gets picky about the wording. Nail a handful of exact sentences and you basically clear the whole topic.
We're going to sort out what "drug" actually means (spoiler: your morning coffee counts), why antibiotics wreck bacteria but do nothing to viruses, and how bacteria go full supervillain and become "resistant". By the end you'll be answering these MCQs on autopilot. Let's get into it 💊
What Even Is a Drug?E15.1
A drug is any substance taken into the body that modifies or affects chemical reactions in the body. That's the whole definition, and it's way wider than you'd think. It doesn't say "illegal", it doesn't say "medicine", it doesn't say "bad for you". If something gets into you and changes a chemical reaction, it's a drug. Full stop.
Break it into three checkpoints: is it taken into the body? Does it change something? Is that change to a chemical reaction in the body? Three yeses = it's a drug.
> 🔑 Key idea: a drug is any substance taken into the body that modifies or affects chemical reactions in the body. Not "illegal", not "medicine only", just that.
Worked Example: Spot the Real DefinitionE15.1
Problem Which is the correct definition of a drug? (A) something only doctors can prescribe, (B) any illegal substance that harms the body, (C) any substance taken into the body that modifies or affects chemical reactions in the body, (D) a medicine that cures bacterial infections.
Step 1 Ditch A: coffee is a drug and nobody needs a prescription for it. Ditch B: paracetamol is legal and helpful, so "illegal + harmful" is wrong.
Step 2 Ditch D too: that only covers medicine-type drugs and forgets caffeine, alcohol, and the rest.
Step 3 C is the exact syllabus definition, wide enough to include everything. The answer is C. Rule of thumb: if an option sneaks in "illegal", "harmful", or "medicine only", it's the trap.
Drugs Are Literally EverywhereE15.1
Because the definition is so broad, loads of everyday stuff counts as a drug. Caffeine in your coffee and energy drinks? Drug. Nicotine in tobacco? Drug. Alcohol in drinks? Drug. Plus the obvious medicinal ones like antibiotics and painkillers. Every single one gets into your body and changes some chemical reaction, that's the only test.
"Medicinal" drugs are the ones taken to treat illness. But whether a drug is helpful, harmful, legal or illegal is a social judgement layered on top, the biology doesn't care about any of that.
> 🔑 Key idea: medicines AND caffeine, alcohol, nicotine are all drugs. Being helpful/legal has nothing to do with the biological definition.
Worked Example: Why Caffeine Is a DrugE15.1
Problem Caffeine is taken in coffee and changes the rate of some reactions in nerve cells. Using the biological meaning, why is caffeine classed as a drug?
Step 1 Run the checklist. Is it taken into the body? Yes, you drink it. Does it change a chemical reaction? Yes, the question literally says it changes the rate of reactions in nerve cells.
Step 2 That's both boxes ticked. You don't need it to be a medicine, illegal, or harmful, none of that is in the definition.
Step 3 So the answer: caffeine is a drug because it's a substance taken into the body (in coffee) that modifies chemical reactions in the body (it changes the rate of reactions in nerve cells).
Antibiotics: Bacteria's Worst NightmareE15.1
An antibiotic is a drug that treats bacterial infections, it kills the bacteria (or stops them growing) so your body can mop up the rest. Penicillin is the OG, discovered by Fleming back in 1928. Antibiotics are a genuine medical glow-up: infections that used to be a death sentence are now a quick course of tablets.
The one fact the exam tests to death: antibiotics only work on bacteria. Not viruses, not your own cells, just bacteria.
> 🔑 Key idea: antibiotics treat bacterial infections. They kill bacteria but do NOT affect viruses.
Worked Example: Who Do Antibiotics Actually Kill?E15.1
Problem Penicillin is an antibiotic. Which type of microorganism do antibiotics kill, viruses, bacteria, all microorganisms, or your own cells?
Step 1 Antibiotics are made to attack bacteria specifically, hitting the cell wall or ribosomes that bacteria have.
Step 2 Not viruses (they lack those parts), not "all microorganisms", and definitely not your own cells (antibiotics are designed to leave those alone).
Step 3 The answer is bacteria. Whenever you see "penicillin" or "antibiotic", instantly think: kills bacteria, ignores viruses.
Antibiotics vs Viruses: A Total No-ShowE15.1
Here's the thing everyone messes up: if your illness is caused by a virus, antibiotics do absolutely nothing. Flu? Virus. Common cold? Virus. Measles? Virus. Taking antibiotics for any of those is like using a key on a door that has no lock, it just doesn't fit.
Bacterial illnesses (cholera, lots of chest and throat infections) can be treated with antibiotics. So step one is always: is this bug a bacterium or a virus?
> 🔑 Key idea: bacterial infection → antibiotics can help. Viral infection (flu, cold, measles) → antibiotics do nothing.
Worked Example: Why Flu Laughs at AntibioticsE15.1
Problem A person has influenza, which is caused by a virus. Explain why antibiotics won't cure this illness.
Step 1 Flu is viral, and antibiotics only work on bacteria, so straight away they're aimed at the wrong target.
Step 2 For the Extended marks, give the reason: a virus has no cell wall, no ribosomes, and no chemical reactions of its own for the antibiotic to attack, and it hides inside your body cells.
Step 3 So there's literally nothing for the antibiotic to hit. It can't kill the virus, so it can't cure the flu. (Your immune system handles it, or a vaccine stops it first.)
Why Viruses Dodge Every AntibioticE15.1
At Extended level you need the why, not just the what. It comes down to how different the two are. A bacterium is a full living cell: it's got a cell wall, ribosomes, and its own metabolism. Antibiotics jam those, like stopping the bacterium building its wall so it bursts. Your cells have no wall and slightly different ribosomes, so the antibiotic hurts the bacterium and leaves you fine.
A virus isn't even a cell. No wall, no ribosomes, no metabolism of its own, and it only copies itself by hijacking your cells from the inside. No target = no effect. That's the whole reason in one line.
> 🔑 Key idea: antibiotics attack bacterial structures (cell wall, ribosomes) that viruses simply don't have. No target, no effect.
Worked Example: Bacterium vs Virus, Side by SideE15.1
Problem Explain, using two differences between a bacterium and a virus, why an antibiotic can kill a bacterium but not a virus.
Step 1 Difference one: a bacterium has a cell wall and ribosomes that antibiotics can attack; a virus has neither.
Step 2 Difference two: a bacterium does its own chemical reactions (metabolism) an antibiotic can disrupt; a virus has no metabolism of its own and hides inside body cells.
Step 3 So the antibiotic has real targets on the bacterium (wall, ribosomes, metabolism) but nothing to aim at on the virus. That's why it kills one and ignores the other.
When Bacteria Fight Back: ResistanceE15.1
Plot twist: antibiotics don't always win. Some bacteria are resistant, meaning the antibiotic doesn't kill them even at the normal dose. A resistant infection just won't clear up the usual way, and doctors have to keep trying different antibiotics (if any still work).
The word you must use is resistant, not "immune". "Immune" is your body's defence system, totally different chapter. Bacteria that shrug off a drug are resistant to it.
> 🔑 Key idea: resistant bacteria are NOT killed by the antibiotic. Use the word "resistant", never "immune".
Worked Example: Name the SurvivorsE15.1
Problem A scientist finds a group of bacteria that are not killed by a normal dose of an antibiotic. What are these bacteria described as?
Step 1 They survived a dose that would normally kill their species, so they're not ordinary bacteria.
Step 2 The exact word for "not killed by the antibiotic" is resistant. Not "immune" (that's your body's defences), not "dead" (obviously).
Step 3 The answer: antibiotic-resistant bacteria.
How Resistance Actually HappensE15.1
Resistance isn't the bacteria "deciding" to fight back, it's natural selection on fast-forward. Four steps, learn them in order: (1) in a big population, a few bacteria are resistant by chance thanks to a random mutation; (2) the antibiotic kills the non-resistant ones but the resistant ones survive; (3) the survivors reproduce and pass on the resistance gene; (4) soon the whole population is resistant, and boom, a strain like MRSA exists.
Key nuance the examiner loves: the antibiotic doesn't create the resistance. The resistant bacteria were already there, the antibiotic just clears out the competition (that's the "selection pressure").
> 🔑 Key idea: variation → antibiotic kills non-resistant → resistant survive → they reproduce → population becomes resistant. The antibiotic selects, it doesn't create.
Worked Example: Read the Survivor PatternE15.1
Problem A population of bacteria is treated with an antibiotic. Most die, but a few survive and multiply to rebuild the population. What does this pattern show?
Step 1 "Most die, a few survive" = the survivors were resistant; the antibiotic killed everyone else but not them.
Step 2 "Survive and multiply to rebuild" = those resistant survivors reproduced and passed on the resistance, so the new population is all resistant.
Step 3 So the pattern shows natural selection producing a resistant population. It does NOT show the antibiotic "making" them resistant, the resistant ones were already in the mix.
MRSA and the Superbug ProblemE15.1
MRSA is the poster child for antibiotic resistance, a strain resistant to loads of antibiotics, which makes hospital infections genuinely dangerous. Since using antibiotics is exactly what selects for resistance, the fix is to use them as little as possible while still treating real bacterial illness. That's the syllabus line: using antibiotics only when essential can limit resistant bacteria such as MRSA.
More antibiotic use = more selection pressure = resistance spreads faster. Less unnecessary use = slower spread. Simple cause and effect.
> 🔑 Key idea: using antibiotics only when essential applies the selection pressure less often, so resistant strains like MRSA develop and spread more slowly.
Worked Example: Why "Only When Essential" WorksE15.1
Problem MRSA is resistant to many antibiotics. Explain why using antibiotics only when they're essential helps to limit strains like MRSA.
Step 1 Every use of an antibiotic kills the non-resistant bacteria and lets the resistant ones survive and reproduce, that's the selection pressure.
Step 2 So the more often antibiotics are used (especially when they're not even needed), the more often that selection happens and the faster MRSA spreads.
Step 3 Using them only when essential applies that pressure less often → fewer resistant bacteria favoured → resistant strains spread more slowly. Less use, slower MRSA.
Using Antibiotics Like a ProE15.1
Two golden habits slow resistance. First: only take antibiotics for real bacterial infections, never for viral colds and flu (they do nothing anyway but still breed resistance in your harmless bacteria). Second: always finish the full course, even when you feel better, so every target bacterium is killed and no tough survivors are left to multiply.
Stopping early is the classic trap answer, it feels responsible but it leaves the hardiest bacteria alive to rebuild. Reducing pointless antibiotic use (like in farming) helps too.
> 🔑 Key idea: only use antibiotics when genuinely needed AND finish the whole course. Stopping early leaves the toughest bacteria to multiply.
Worked Example: Pick the Best PlanE15.1
Problem A health service wants the best long-term plan to slow resistant bacteria. Which pair works best? (A) antibiotics for every infection + stop early when you feel better, (B) only for genuine bacterial infections + finish the full course, (C) a different antibiotic every day, (D) antibiotics for healthy people to prevent illness.
Step 1 A is wrong twice: "every infection" includes viral ones (pointless use), and stopping early leaves survivors.
Step 2 C and D both crank up antibiotic use, which increases selection for resistance. Wrong direction entirely.
Step 3 B has the two real fixes: prescribe only for genuine bacterial infections (less selection) and finish the course (kill every target). The answer is B.
Key Points to Remember
- A drug is any substance taken into the body that modifies or affects chemical reactions in the body, includes caffeine, alcohol, nicotine, not just medicines.
- Antibiotics treat bacterial infections (e.g. penicillin). They kill bacteria but do NOT affect viruses.
- Extended reason viruses are unaffected: a virus has no cell wall, no ribosomes, and no metabolism of its own to attack, and it hides inside body cells.
- Some bacteria are resistant (not killed by the antibiotic). Use the word "resistant", not "immune".
- Resistance develops by natural selection: a few resistant by chance → antibiotic kills the non-resistant → resistant survive and reproduce → population becomes resistant (e.g. MRSA).
- Using antibiotics only when essential, and finishing every course, limits resistant strains by applying the selection pressure less often.