Biology · 15 Drugs

15.1 Drugs

Before you start

A pathogen is an organism that causes disease; bacteria and viruses are both types of pathogen.
Bacteria are living cells; viruses are not cells and can only reproduce inside a host cell.
Bacteria reproduce very quickly when conditions are suitable.
A mutation is a random change to the genetic material, and mutations are one source of variation.
Percentage = (part ÷ total) × 100, and you may use a calculator in every paper.

Syllabus objectives covered (5)

Describe a drug as any substance taken into the body that modifies or affects chemical reactions in the body
Describe the use of antibiotics for the treatment of bacterial infections
State that some bacteria are resistant to antibiotics which reduces the effectiveness of antibiotics
State that antibiotics kill bacteria but do not affect viruses
Explain how using antibiotics only when essential can limit the development of resistant bacteria such as MRSA

Part 1 of 5

What actually counts as a drug

Start with the definition, because this one is marked almost word for word: a drug is any substance taken into the body that modifies or affects chemical reactions in the body.
Now read it again and notice what is not in it. It does not say medicine. It does not say illegal. It does not say harmful. None of those ideas is part of the definition.
So the caffeine in your tea is a drug. Alcohol is a drug. An antibiotic is a drug. A painkiller is a drug. What makes something a drug is what it does to the chemistry inside you, not where you buy it.
Two phrases carry the marks. The first is taken into the body — the substance has to get inside you. The second is modifies or affects chemical reactions — and that exact idea, chemical reactions, is the phrase examiners look for.
If you write "a drug is something that changes how your body works", you have described the right thing in the wrong words and you are relying on the examiner's goodwill. Write "affects chemical reactions in the body" and there is nothing to argue about.
The trap: writing "a drug is a medicine that cures illness". That scores nothing. It is far too narrow — it excludes alcohol and caffeine — and it names a purpose rather than an effect.

Part 2 of 5

Antibiotics: what they are used for

An antibiotic is a drug used to treat infections caused by bacteria. That is the whole of its job description.
Here is the sequence you are describing when a question asks how antibiotics are used. A doctor decides the infection is bacterial. An antibiotic is prescribed. The antibiotic kills the bacteria inside the patient. With the bacteria gone, the infection clears and the symptoms stop.
Notice that the antibiotic treats the cause, not the symptoms. A painkiller makes your throat hurt less; an antibiotic removes the bacteria that were making it sore in the first place. Questions sometimes hang on that difference.
You are not examined on the molecular mechanism — how an antibiotic gets inside a bacterial cell and interferes with it is beyond this syllabus. What you are examined on is what antibiotics are used for, what they kill, and what they leave completely untouched.
Say kill bacteria, not "fight infection" or "get rid of germs". "Germs" is not a scientific word and will not be credited, and "fights" does not say what happens to the bacteria.

Part 3 of 5

Why antibiotics do nothing at all to viruses

One sentence to be able to state cold: antibiotics kill bacteria but do not affect viruses.
The reason sits in what a virus is. A bacterium is a living cell with its own structures and its own chemical reactions going on inside it, and an antibiotic interferes with those.
A virus is not a cell. It has no cell structures of its own carrying out reactions, and it reproduces inside your own cells, using your cells' machinery. There is simply nothing there for an antibiotic to act on.
So the illnesses caused by viruses — colds, influenza, measles, chickenpox — cannot be treated with antibiotics. Not treated slowly. Not treated less well. Not treated at all.
That last point is a real marking distinction. "Antibiotics are less effective against viruses" is wrong, because it implies some effect. The credited wording is that they have no effect on viruses, or that they do not affect viruses.
A useful check when you meet an unfamiliar disease in a question: the stem will always tell you whether the pathogen is a bacterium or a virus. You are never expected to know from memory which category a rare disease falls into — read the stem and use what it gives you.

Part 4 of 5

Resistant bacteria and what they cost us

Some bacteria are resistant to an antibiotic. Resistant means exactly this: the antibiotic does not kill them. They survive it and carry on reproducing.
Resistance belongs to the bacterium, never to the patient. This is the single most common wording error in the whole topic. "The person has become immune to the antibiotic" scores zero — and it is a genuinely different idea, because immunity is about your white blood cells recognising a pathogen, not about a drug failing.
The word resistant is load-bearing. "Immune to the antibiotic", "used to the antibiotic" and "stronger than the antibiotic" are all rejected. Use resistant, and attach it to the bacteria.
Now the consequence, which is the second half of the objective. If some of the bacteria causing an infection are resistant, the antibiotic kills only the non-resistant ones. The resistant ones remain, the infection is not cleared, and the patient stays ill.
Said in exam language: the presence of resistant bacteria reduces the effectiveness of the antibiotic. The antibiotic itself has not changed — the bacteria it is being used against have.
MRSA is the named example in your syllabus: a strain of bacterium that is resistant to several antibiotics and is a particular problem in hospitals. Learn the name — it is the one you are expected to be able to produce.

Part 5 of 5

How resistance builds up, and why 'only when essential' works

Build this in the order it happens, because the marks are awarded for the steps, not for the conclusion.
Step 1 — the variation is already there. Bacteria in a population are not all identical. Mutation happens by chance, and by chance a mutation can produce a bacterium with an allele that makes it resistant to a particular antibiotic.
That chance mutation happens whether or not any antibiotic is present. Hold on to this: the antibiotic does not create resistance. It only decides who survives.
Step 2 — the antibiotic is used. Every bacterium that is not resistant is killed. Every bacterium that is resistant survives.
Step 3 — the survivors reproduce. Bacteria reproduce very quickly, and each resistant survivor passes the allele for resistance on to its offspring.
Step 4 — the population has changed. The proportion of the bacteria that are resistant is now much higher than before. Repeat this often enough and almost the whole population is resistant, which is how a strain like MRSA arises and spreads.
Now the Extended point. Each of those steps only runs when an antibiotic is actually used. The antibiotic is what gives resistant bacteria their advantage — with no antibiotic present, being resistant is no help at all.
So if antibiotics are used only when they are essential, resistant bacteria are handed that advantage far less often. Fewer rounds of killing off the non-resistant bacteria means the resistant proportion rises far more slowly, and resistant strains such as MRSA develop and spread more slowly.
Here is the case that makes it concrete. A patient with a viral sore throat is given an antibiotic. It cannot cure the sore throat, because antibiotics do not affect viruses. But it still kills off the non-resistant bacteria living harmlessly in that patient, leaving the resistant ones behind. The only lasting result of that prescription is more resistance. That is precisely the use that is not essential.
When the question is worth four marks, examiners are looking for the chain, not the headline: variation or mutation → non-resistant bacteria killed → resistant bacteria survive → they reproduce and pass on the resistance → the proportion of resistant bacteria increases. Write it as a chain and each link picks up its mark.

Worked examples

Example 1. A student says, "Caffeine cannot be a drug, because you can buy coffee in any shop." Explain whether the student is correct.

Show the working

Do not argue about coffee yet. Go straight to the definition, because the definition settles it: a drug is any substance taken into the body that modifies or affects chemical reactions in the body.
Test caffeine against each half. Is it taken into the body? Yes — you drink it. Does it modify or affect chemical reactions in the body? Yes — that is why it changes how alert you feel.
Now deal with the student's actual reason. Where a substance is sold, and whether it is legal, appear nowhere in the definition, so they cannot decide the question.
State the conclusion in the definition's own words rather than your own, so the marking point is unmistakable.

Answer: The student is not correct. Caffeine is a drug because it is a substance taken into the body that affects chemical reactions in the body. Whether a substance is sold freely or is legal is not part of the definition of a drug.

Example 2. A patient has influenza, which is caused by a virus. The patient asks for an antibiotic. Explain why the doctor does not prescribe one. [3]

Show the working

Three marks means three separate points, so plan three before you write one.
Point one: identify the pathogen from the stem. Influenza is caused by a virus, not a bacterium.
Point two: state what antibiotics act on. Antibiotics kill bacteria.
Point three: state what they do to viruses — no effect. Say it absolutely, not "less effect", so the mark cannot be refused.
Finish with the consequence for this patient: the antibiotic would not cure the illness. That ties the three facts to the doctor's actual decision.

Answer: Influenza is caused by a virus. Antibiotics kill bacteria but they do not affect viruses, so an antibiotic would have no effect on the virus and would not cure the patient's illness.

Example 3. A hospital tests samples of one species of bacterium taken from patients and records how many are resistant to a particular antibiotic. In 2015, $8$ of $200$ samples were resistant. In 2023, $63$ of $210$ samples were resistant. Calculate the percentage of samples that were resistant in each year, and explain the change.

Show the working

Two different totals were tested, so the raw counts cannot be compared directly. Convert each to a percentage first — that is why the question gives you both numbers.
$percentage resistant = \frac{number resistant}{number tested} \times 100$
For 2015, divide $8$ by $200$ and multiply by $100$ .
$\frac{8}{200} \times 100 = 4$
For 2023, divide $63$ by $210$ and multiply by $100$ .
$\frac{63}{210} \times 100 = 30$
So resistance rose from $4$ % to $30$ %. Now explain it, and explain it as a chain rather than as one sentence.
Variation first: some of these bacteria already carried an allele for resistance, produced by a chance mutation.
Then the selection: each time the antibiotic was used in the hospital it killed the non-resistant bacteria, and only the resistant ones survived.
Then the increase: those survivors reproduced and passed the resistance on to their offspring, so the proportion of resistant bacteria climbed year on year.

Answer: $4$ % of samples were resistant in 2015 and $30$ % in 2023. Some bacteria were resistant because of a chance mutation. Each use of the antibiotic killed the non-resistant bacteria, so only the resistant ones survived, reproduced and passed on the resistance, increasing the proportion of resistant bacteria in the hospital.

Where people go wrong

Mistake: Expecting antibiotics to treat a cold, influenza or any other viral illness — and writing that they are "less effective" against viruses.

Instead: Antibiotics kill bacteria but do not affect viruses. There is no effect at all, not a reduced one.

A virus is not a cell. It has no structures or chemical reactions of its own for the antibiotic to interfere with, and it reproduces inside your own cells. "Less effective" implies some effect and is marked wrong.

Mistake: Writing that a person has "become immune to antibiotics" or that their body "got used to the antibiotic".

Instead: The bacteria become resistant, not the person. Say "the bacteria are resistant to the antibiotic".

Resistance is a property of the bacterium. Immunity is something different — it is your white blood cells recognising a pathogen. Naming the wrong organism loses the mark even when the rest of the answer is right.

Mistake: Saying the antibiotic causes the bacteria to mutate, or that bacteria "learn to survive" the antibiotic.

Instead: The mutation happens by chance and is already present in some bacteria. The antibiotic only selects which bacteria survive.

Nothing about the antibiotic changes a bacterium's genetic material. It kills the non-resistant ones, and the resistant ones that were already there go on to reproduce. Getting this backwards costs the first mark of every four-mark explanation.

Mistake: Assuming "drug" means an illegal substance, so a prescribed antibiotic is "a medicine, not a drug".

Instead: A drug is any substance taken into the body that modifies or affects chemical reactions in the body. Medicines are drugs.

Legality and where a substance is sold appear nowhere in the definition. An answer built on that distinction is answering a question about society, not about biology.

Mistake: Explaining resistance by saying the antibiotic "gets weaker" or "stops working over time".

Instead: The antibiotic is unchanged. What changes is the bacterial population, which now contains a much higher proportion of resistant bacteria.

The objective is that resistant bacteria reduce the effectiveness of antibiotics — the cause sits in the bacteria. An answer that puts the change in the drug describes the wrong thing changing.

Check yourself

Define the term drug.
Answer
Any substance taken into the body that modifies or affects chemical reactions in the body.
What type of infection is an antibiotic used to treat, and what does it do to the pathogen?
Answer
An infection caused by bacteria. The antibiotic kills the bacteria, so the infection is cleared.
A friend with a cold says the antibiotics she has been given will "work eventually". What is wrong with that?
Answer
A cold is caused by a virus. Antibiotics kill bacteria but do not affect viruses, so they will have no effect at all, however long she takes them.
State what is meant by saying that a bacterium is resistant to an antibiotic, and what this means for the patient.
Answer
The antibiotic does not kill it, so it survives and continues to reproduce. The antibiotic is less effective, so the infection is not cleared.
Give the name of a bacterial strain that is resistant to several antibiotics and is a particular problem in hospitals.
Answer
MRSA.
Put these four stages of the development of antibiotic resistance in order: resistant bacteria survive and reproduce; a chance mutation makes a bacterium resistant; the proportion of resistant bacteria increases; the antibiotic kills non-resistant bacteria.
Answer
A chance mutation makes a bacterium resistant → the antibiotic kills the non-resistant bacteria → the resistant bacteria survive and reproduce, passing on the resistance → the proportion of resistant bacteria increases.
Explain how prescribing antibiotics only when they are essential slows the development of resistant bacteria such as MRSA.
Answer
Resistant bacteria only gain an advantage when an antibiotic is present, because that is when the non-resistant bacteria are killed. Using antibiotics less often means this selection happens far less often, so the proportion of resistant bacteria rises much more slowly and resistant strains such as MRSA spread more slowly.
Is an antibiotic a drug? Justify your answer using the definition.
Answer
Yes. It is taken into the body and it affects chemical reactions there, which is exactly the definition of a drug. It is a drug used to treat bacterial infections.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. Two patients visit the same clinic on the same morning. Patient A has a chest infection caused by a bacterium. Patient B has a cold caused by a virus. The doctor prescribes an antibiotic for patient A only.

(a) Define the term drug. 2 marks No calculator

Mark scheme

B1 any substance taken into the body
B1 that modifies / affects chemical reactions in the body

(b) Explain why the doctor prescribed an antibiotic for patient A but not for patient B. 3 marks No calculator

Mark scheme

B1 patient A's infection is caused by a bacterium / bacteria
B1 antibiotics kill bacteria, so the infection is treated
B1 patient B's illness is caused by a virus and antibiotics do not affect viruses, so it would not cure the cold, owtte

(c) Patient B asks whether the antibiotic would work if she took it for longer. State the effect that taking the antibiotic for longer would have on the virus. 1 mark No calculator

Mark scheme

B1 no effect / none, however long it is taken (ignore references to side effects)

Question 2. A hospital records the number of samples of one species of bacterium that are resistant to a particular antibiotic. In 2014, 12 of 400 samples tested were resistant. In 2022, 96 of 320 samples tested were resistant.

(a) Calculate the percentage of the samples tested in 2014 that were resistant to the antibiotic. 2 marks

Mark scheme

C1 12 / 400 × 100 seen, in any form
A1 3 %

(b) In 2022, 30 % of the samples tested were resistant. Explain how the proportion of resistant bacteria in the hospital increased between 2014 and 2022. 4 marks No calculator

Mark scheme

B1 variation: some bacteria had a mutation / allele making them resistant, arising by chance
B1 when the antibiotic was used, the bacteria that were not resistant were killed
B1 the resistant bacteria survived and reproduced
B1 the resistance was passed on to the offspring, so the proportion of resistant bacteria increased (CON: any statement that the antibiotic caused the mutation)

(c) Suggest two changes the hospital could make to slow the further development of resistant bacteria. 2 marks No calculator

Mark scheme

B1 prescribe antibiotics only when they are essential / reduce unnecessary prescriptions
B1 do not prescribe antibiotics for infections caused by viruses, since they have no effect on them

Question 3. MRSA is a strain of bacterium that is resistant to several antibiotics.

(a) State what is meant by describing MRSA as resistant to an antibiotic. 1 mark No calculator

Mark scheme

B1 the antibiotic does not kill it / it is not affected by the antibiotic and survives (ignore 'immune')

(b) Explain why an infection caused by MRSA is more difficult to treat than an infection caused by a strain that is not resistant. 2 marks No calculator

Mark scheme

B1 the antibiotics normally used do not kill MRSA, so the infection is not cleared
B1 a different antibiotic, to which the strain is not resistant, has to be found and used, ORA

How this is examined

How a population of bacteria comes to be mostly resistant, told as a sequence: a four-mark Explain, usually attached to a table or graph showing the percentage of resistant samples rising over several years Four separate links, each in its own clause: (1) some bacteria have a mutation / allele making them resistant, and this arose by chance; (2) the antibiotic kills the bacteria that are not resistant; (3) the resistant bacteria survive and reproduce; (4) the resistance is passed on to the offspring, so the proportion of resistant bacteria increases. Writing 'the bacteria became resistant to the antibiotic' as one sentence scores one mark at most, because it collapses all four links into a claim.
Why a doctor refuses an antibiotic for a viral illness: a two- or three-mark Explain, in a stem that tells you the illness is caused by a virus and then asks why the antibiotic will not help Say all three: the illness is caused by a virus; antibiotics kill bacteria; antibiotics do not affect viruses, so it would not cure the illness. The absolute wording is what scores — 'do not affect viruses' or 'have no effect on viruses'. 'Are less effective against viruses' is marked wrong because it implies some effect.
Why cutting unnecessary prescriptions slows resistance: a two- or three-mark Explain or Suggest, often the last part of a question about MRSA in a hospital The link between using the drug and the advantage it confers: the antibiotic is what kills the non-resistant bacteria, so it is the antibiotic's use that gives resistant bacteria their advantage; using antibiotics only when essential means that selection happens less often, so resistant strains such as MRSA develop and spread more slowly. Naming MRSA when the question mentions it costs nothing and shows you are answering the case in front of you.
The definition of a drug, stated in the syllabus's own words: a one- or two-mark Define or State, often the opening part of a longer question on medicines Both halves: 'a substance taken into the body' and 'that modifies or affects chemical reactions in the body'. Naming a purpose ('a medicine that cures disease') or a category ('an illegal substance') earns nothing.
What 'resistant' means, and whose property it is: a one-mark State, or a marking point buried inside a longer Explain where the wrong wording throws away a mark you had otherwise earned 'The antibiotic does not kill the bacterium', with resistance attached to the bacteria. 'The person becomes immune to the antibiotic' is rejected, as is 'the antibiotic gets weaker' — the antibiotic is unchanged, the bacterial population is what has changed.
Turning counts of resistant samples into percentages before comparing them: a two-mark Calculate that comes before the Explain, where two years have different numbers of samples tested Showing number resistant ÷ number tested × 100 and giving the answer with a % sign. Comparing the raw counts without converting them is the trap the question is built around, since the totals tested differ.

Words to know

Drug: Any substance taken into the body that modifies or affects chemical reactions in the body.
Antibiotic: A drug used to treat infections caused by bacteria. It kills bacteria but does not affect viruses.
Resistant: Describes a bacterium that is not killed by a particular antibiotic, so it survives the treatment and continues to reproduce.
MRSA: A strain of bacterium that is resistant to several antibiotics and is a particular problem in hospitals.
Mutation: A random change in the genetic material. In bacteria it can produce an allele that makes the bacterium resistant to an antibiotic.
Pathogen: An organism that causes disease. Bacteria and viruses are both pathogens, but only bacterial infections can be treated with antibiotics.

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