Health and drugs · System
What drugs do to the body
Caffeine, paracetamol, nicotine and alcohol are all drugs. That is not a judgement about any of them — it is a description of what the molecules do once they are in your blood.
Start here
A mug of coffee contains a drug.
Around 90 milligrams of caffeine, bought in a café, legal at any age, no prescription. It changes how your nerve cells behave, and that is why you drink it. So the word drug cannot mean what most people think it means.
What makes a substance a drug?
A drug is any substance that changes the way the body works. That is the whole definition. Whether it is legal, whether a doctor prescribed it, and whether it is addictive are three separate questions, and the answers do not line up the way you would expect.
At the bench · one dose, one bloodstream
Follow the dose
not started
Pick a drug and follow one dose of it, one step at a time. Watch step 3 carefully — it is the step everyone skips, and it is the reason side effects exist at all.
The dose
Caffeine
Paracetamol
Nicotine
Alcohol (ethanol)
Stimulant
Painkiller
Stimulant, strongly addictive
Depressant
Coffee, tea, cola, energy drinks. Legal at any age, sold in a café.
A legal medicine, bought without a prescription, with a dose limit printed on the box.
Cigarettes and vapes. Legal to sell to adults in the UK, illegal to sell to under-18s.
Beer, wine, spirits. Legal for adults in the UK, with age limits on buying it.
- InSwallowed in a drink. It crosses the wall of the small intestine into the blood within a few minutes.
- Into the bloodThe drug is now dissolved in the blood plasma. Nothing about the molecule says where it is supposed to go.
- Once round the whole bodyBlood makes the full circuit in under a minute. Every organ you have is offered this drug, whether it needs it or not. There is no address on it and no way to send it to one place.
- It acts where it fitsIt reaches the brain and blocks the chemical signal that builds up through the day to make you feel sleepy. Nerve cells go on firing readily, so you feel alert.
- And everywhere else it reachedListed below — and none of them is a fault in the drug.
- InSwallowed as a tablet. It dissolves in the stomach and is absorbed through the small intestine wall.
- Into the bloodThe drug is now dissolved in the blood plasma. Nothing about the molecule says where it is supposed to go.
- Once round the whole bodyBlood makes the full circuit in under a minute. Every organ you have is offered this drug, whether it needs it or not. There is no address on it and no way to send it to one place.
- It acts where it fitsIt acts on the pathways carrying pain signals, and on the temperature control centre of the brain. It reduces the pain and brings a fever down. It does nothing whatever to the injury or infection causing them.
- And everywhere else it reachedListed below — and none of them is a fault in the drug.
- InInhaled. It crosses the thin alveoli walls straight into the blood and reaches the brain in about ten seconds — faster than any swallowed drug.
- Into the bloodThe drug is now dissolved in the blood plasma. Nothing about the molecule says where it is supposed to go.
- Once round the whole bodyBlood makes the full circuit in under a minute. Every organ you have is offered this drug, whether it needs it or not. There is no address on it and no way to send it to one place.
- It acts where it fitsIt triggers a release of the brain’s own reward chemical. The brain adapts to expect that release, so the wanting becomes a physical state of the nervous system rather than a weakness of character. That adaptation is what addiction is.
- And everywhere else it reachedListed below — and none of them is a fault in the drug.
- InSwallowed. Unusually, it is absorbed straight through the stomach wall as well as through the intestine, so it arrives in the blood quickly.
- Into the bloodThe drug is now dissolved in the blood plasma. Nothing about the molecule says where it is supposed to go.
- Once round the whole bodyBlood makes the full circuit in under a minute. Every organ you have is offered this drug, whether it needs it or not. There is no address on it and no way to send it to one place.
- It acts where it fitsIt slows the signals passing between nerve cells throughout the brain. Reactions lengthen, coordination goes, and judgement goes first — which is the dangerous part, because poor judgement is the thing least able to notice itself.
- And everywhere else it reachedListed below — and none of them is a fault in the drug.
Where else the same dose went
Heart
Beats faster. Two mugs is enough to measure the difference.
Kidneys
Produce more urine, which is why coffee sends you to the toilet.
Stomach
Releases more acid, which is why coffee on an empty stomach can hurt.
None of those three organs has anything to do with feeling awake. They were simply on the route.
Where else the same dose went
Liver
Breaks the drug down, at a fixed rate it cannot exceed. Too much in one day damages liver cells, and that damage can be permanent even though the person feels fine for a day or two.
Kidneys
Filter the broken-down drug out into the urine. This is how the dose eventually leaves you.
Every other organ
Received the drug and had no use for it.
The tablet did not travel to your head. It went everywhere, and your head is where there was something for it to act on.
Where else the same dose went
Heart
Beats faster, and blood vessels narrow, so blood pressure rises.
Blood vessels in the skin
Narrow too, reducing blood flow to the surface — one reason wounds heal more slowly in smokers.
Airways and lungs
Damaged by tar and by the other substances in smoke rather than by nicotine itself. Which substance does which damage is the subject of Exercise, asthma and smoking.
Nicotine is the reason people keep smoking. The substances that cause most of the harm are different molecules travelling in the same smoke.
Where else the same dose went
Liver
Breaks it down at roughly one unit an hour, and nothing raises that rate. Years of heavy drinking scar the liver permanently.
Kidneys
Produce more urine, so heavy drinking dehydrates you.
Stomach
Lining irritated directly, which is why heavy drinking causes sickness.
A depressant does not make you sad — it slows nerve signals down. That is a statement about signalling speed, not about mood.
Sorted by what they do, not by what they are called
Three classes, and all three are about nerve signals.
Stimulants · Speeds signals up
Nerve signals pass more readily, so heart rate and alertness rise. Caffeine, nicotine. Prescribed stimulants exist too, and they are prescribed precisely because the effect is useful at a controlled dose.
Depressants · Slows signals down
Signals between nerve cells pass less readily, so reactions, coordination and judgement all worsen. Alcohol, and the sedatives used in anaesthetics. Slower is not safer: most alcohol harm comes from what a slowed nervous system then does.
Painkillers · Blocks the message
The pain signal is reduced or blocked on its way to the brain. The cause of the pain is untouched. Paracetamol, ibuprofen, and morphine at the strong end. Pain that stops being felt has not stopped being a warning.
Key fact
A drug is a substance that changes the way the body works. The blood carries it to every organ, so it acts wherever it fits — which is why a drug taken for one job also does other things you did not ask for.
Think again
“Drugs are illegal substances.”
Legality is a decision made by a parliament. It differs between countries, it has changed several times within living memory, and the molecule is unaffected by any of it. Caffeine is a drug and it is sold to children. Paracetamol is a drug and it is in most kitchen drawers. Alcohol and nicotine are drugs, they are legal for adults in the UK, and between them they cause more illness and more deaths here than every illegal drug combined. Meanwhile morphine is a controlled drug that is also given in hospitals every day. So legal and safe are not the same word, illegal and harmful are not the same word, and drug and medicine describe overlapping sets rather than opposite ones: a medicine is simply a drug used to treat something.
“A painkiller goes to the part that hurts.”
Nothing in a tablet knows where your head is. It dissolves, crosses the gut wall into the blood, and the blood takes it round the entire body in under a minute — liver, kidneys, skin, feet, all of it, whether they need it or not. The tablet works on your headache because the pain pathways carrying that particular signal are something it can act on, not because it travelled there. Everything else it touched on the way is what we call a side effect, and side effects are not a manufacturing fault. They are the unavoidable consequence of delivering a drug through a system that goes everywhere. It also explains why the number on the box matters: your liver breaks the drug down at a fixed rate, and a second dose taken early does not get a second liver.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · What the word means
Which statement is true of every drug — prescribed, bought in a shop, or illegal?
Rung 2 · The one that catches people
You swallow a paracetamol tablet for a headache. Where does the drug go?
Rung 3 · Explain a side effect
A friend takes a caffeine tablet to stay awake revising and finds their heart is racing. They think they have had a bad reaction to a faulty tablet. Explain what actually happened, using the route the drug took.
Rung 4 · Take it somewhere new
Someone has a bad headache. Two paracetamol have not worked after an hour, so they decide to take two more early, reasoning that a bigger dose must work better. Using what you know about the liver and about dose, explain the risk — and say what they should do instead.
Key note
A drug is any substance that changes how the body works. Stimulants speed nerve signals up, depressants slow them down, painkillers block the pain signal without touching its cause. Every drug is carried everywhere by the blood, acts where it fits, and is broken down mainly by the liver at a rate that cannot be hurried. Dose decides whether a drug treats you or harms you.
Going further
Digoxin comes from the foxglove in the hedgerow. At the right dose it steadies a failing heart and it has been prescribed for two hundred years; a little above that dose it stops the heart, which is why foxglove is one of the most dangerous plants in Britain. Same molecule, same body, different amount — so poison is not a category of substance at all, it is a statement about quantity. Even water follows the rule: drink several litres in an hour and you can dilute your blood dangerously. This is why pharmacists talk about doses rather than about safe and unsafe substances, and why it is natural, so it must be gentle is a claim about where a molecule came from rather than about what it does.
If any of this is about you or someone you know
This lesson covers the biology only. Decisions, pressure from other people and the law belong to your PSHE and RSE lessons, which is where to raise them. For anything about your own health, a medicine you are taking, or someone you are worried about, talk to a trusted adult, the school nurse, a pharmacist or a GP — a pharmacist will answer a question about a medicine without an appointment. Your school's PSHE materials list the national confidential services by name.
Before this lesson
Connects to
At GCSE this becomes
- Drug development and testing: preclinical trials, placebos, double-blind trials, and how a therapeutic dose is decided.
Where to next
- Next: Alcohol and smoking
- Previous: Seed dispersal
Reproduction
Ask Mr Badmus AI
Want to talk through why side effects are unavoidable rather than a fault?
If any of this is about you or someone you know, talk to someone you trust — a parent or carer, a teacher, your school nurse, a pharmacist or your GP. Out of school hours: Childline — 0800 1111, free and confidential. FRANK — 0300 123 6600.
The tracer is a simplified model. It follows one dose of one drug through five stages in order, and gives no doses, thresholds or timings for any substance. Real absorption, distribution and breakdown overlap in time and differ between people. Nothing on this page is medical advice.
Lesson content © MrBadmusAI.