Drug — taken into the body, alters its chemistry
Both halves of the definition earn marks: (1) any substance taken into the body, and (2) one that modifies or affects chemical reactions in the body. The definition is deliberately broad — it covers medicinal drugs such as antibiotics and any other substance that changes body chemistry once inside. Answers such as "a drug is medicine" or "a drug makes you feel different" contain neither element and are not credited.
Antibiotics — kill bacteria, no effect on viruses
Antibiotics are drugs used for the treatment of bacterial infections: they kill bacteria, or stop them multiplying. They do not affect viruses, so a cold or influenza cannot be cleared with one. The reason is structural — a bacterium is a living cell with its own biochemical machinery for the antibiotic to disrupt, while a virus is not a cell and offers no such target. State it that precisely; "antibiotics fight infection" is not the syllabus statement.
Antibiotic resistance — the Core fact and the Extended explanation
Core: some bacteria are resistant to an antibiotic — it no longer kills them or stops them multiplying — which reduces the effectiveness of that antibiotic. MRSA (methicillin-resistant Staphylococcus aureus) is the named example. Extended only: using antibiotics only when essential limits the development of resistant bacteria, because each unnecessary or incomplete use lets resistant bacteria survive, reproduce and pass their resistance on.
Drawn from real examiner reports.
A drug is not just a medicine
The mark scheme wants two elements: the substance is taken into the body, and it modifies or affects chemical reactions in the body. Everyday definitions — "medicine you take when you are ill", "something that changes your mood" — contain neither. Dropping either half, in either direction, costs the definition mark.
Digest B14 entry (Jun 2022 / Nov 2022 / Jun 2023 / Nov 2023): "(Thin topic; few report findings.) ... general recall only in these sittings." No sitting- or question-specific citation for the drug definition exists in the digest; this trap is grounded in that general "recall only" finding — marks here are lost on imprecise recall, not on reasoning.
Antibiotics do not treat colds or flu
Treating "antibiotic" as a general word for infection medicine leads candidates to say a cold or influenza can be cleared with one. Antibiotics kill bacteria but do not affect viruses — a virus is not a cell, so there is no machinery for the drug to disrupt. Name the pathogen type; a hedge such as "antibiotics do not always work" earns nothing.
Digest B14 entry (general, four sittings 2022-23): "Antibiotics treat bacterial infections and kill bacteria but not viruses; overuse breeds resistant bacteria (e.g. MRSA)" is recorded as the core recall content for this thin topic, with no sitting-specific question cited.
Conclusion given without the mechanism
The Extended "explain" question wants the selection chain, not the outcome. "Fewer antibiotics means less resistance" states the conclusion and stops. Full marks need: resistant bacteria already exist; unnecessary or incomplete use kills only the non-resistant ones; the survivors reproduce and pass resistance on; so strains such as MRSA spread.
Digest cross-cutting note (introduction, all four sittings, all sciences): "not answering the actual command word (describe vs explain vs state)" is repeatedly flagged as a mark-losing failure across all 0654 papers — applied here to the "explain" command word in Extended statement 5.
Antibiotics select, they do not cause
Using an antibiotic does not make bacteria mutate. Resistant bacteria already exist by chance in the population; the antibiotic kills the non-resistant ones and so selects for the survivors, which then reproduce without competition. Writing that the antibiotic "makes the bacteria resistant" reverses cause and effect and is not credited.
Antibiotic is not a word for any drug
A question about antibiotics is not a question about medicines in general. Antiseptics and painkillers are drugs but not antibiotics, and only antibiotics carry the syllabus statement about killing bacteria and not affecting viruses. Broadening the answer to "drugs" or "medicines" throws away the precision the mark scheme is testing.
Feeling better does not mean cured
Stopping a course early because the symptoms have gone leaves bacteria alive, and the ones surviving longest are the most resistant. They then reproduce without competition and pass resistance on. Completing the full course is more likely to kill the partially resistant bacteria before they can multiply.
Answer the command word
"State" and "describe" want the exact syllabus wording — "modifies or affects chemical reactions in the body", not "changes the body somehow". "Explain" is the only command in this leaf that wants a causal chain rather than recall.
The resistance chain to reuse
Cause: resistant bacteria already exist. Mechanism: unnecessary or incomplete antibiotic use kills the non-resistant ones. Effect: the resistant survivors reproduce and pass resistance on. Consequence: strains such as MRSA become more common, so restrict use to when essential.
One marking point per mark
B14.1 is thin and fact-dense, so tariffs map cleanly onto points: a 2-mark definition needs both elements, a 3-mark antibiotics question needs three separate statements, and the 4-mark "explain" needs four linked steps of the chain.
Name MRSA when an example is asked for
MRSA (methicillin-resistant Staphylococcus aureus) is the one antibiotic-resistant bacterium named in the 0654 syllabus. Give it by name whenever a question wants an example of resistance — a generic "some bacteria" will not do.
What are antibiotics used to treat, and what do they do to bacteria vs viruses?
Describe what is meant by "a drug". (2 marks)
