Drug — the two-element definition
Drug = any substance taken into the body that modifies or affects chemical reactions in the body. Both elements are needed for the mark: (1) taken into the body; (2) modifies/affects chemical reactions. The definition is deliberately neutral, covering prescribed medicines such as antibiotics and painkillers, over-the-counter medicines, and recreational or misused substances — legal and illegal alike.
Antibiotic — kills or inhibits bacteria only
Antibiotic = a substance, e.g. penicillin from the fungus Penicillium, that kills or inhibits the growth of bacteria. It acts on targets unique to bacterial cells: cell wall synthesis, and bacterial-type ribosomes/protein synthesis. Viruses have neither, and replicate using the host cell's own machinery, so an antibiotic has nothing bacteria-specific to disrupt — hence no effect on viral infections.
Antibiotic resistance — natural selection in four links
Antibiotic resistance spreads by natural selection. (1) A random mutation, already present in a few bacteria before treatment, happens to confer resistance. (2) The antibiotic kills the non-resistant bacteria. (3) The resistant bacteria survive and reproduce, passing on the resistance allele. (4) The proportion of resistant bacteria rises over generations. The antibiotic is a selective pressure, never the cause of the mutation. (Extended)
Drawn from real examiner reports.
Drug ≠ illegal substance
Candidates narrow drug to mean only illegal or recreational substances — "something taken to get high". The mark-scheme definition is neutral and has two elements: a substance taken into the body, and one that modifies or affects chemical reactions in it. Prescribed antibiotics and painkillers are drugs too, and "a type of medicine" is equally too narrow.
Drug definition confused with "medicine" or specifically "illegal substance": w22 Paper 31 Q5c.
Antibiotics do not treat viruses
Claiming antibiotics can cure influenza, a cold or HIV is one of the most persistently penalised errors in 0610. Antibiotics kill or inhibit bacteria only. State both halves: effective against bacterial infections, no effect on viral infections. The mirror-image slip — saying bacteria become resistant to antiviral treatments — is flagged too.
Antibiotics stated to treat viral infections: s23 Paper 11 Q30; w22 Papers 11/21 Q27/Q29.
"Viruses are too small" earns nothing
Explaining that antibiotics fail on viruses because "viruses are too small" — or stronger, or more dangerous — is not a credited reason. The marked explanation names a bacteria-specific target the virus lacks: no cell wall, no bacterial-type ribosomes, and replication carried out inside the host cell using the host's own machinery.
The antibiotic does not cause the mutation
Writing that "the antibiotic makes the bacteria mutate" implies the drug rewrites bacterial DNA on exposure — a Lamarckian, directed-mutation idea that scores zero for this element. Mutations arise randomly and spontaneously during bacterial reproduction and already exist in the population before any antibiotic is used.
Misconception that the antibiotic triggers the mutation: w22 Paper 41 Q4b i.
Bacteria do not mutate on purpose
Answers saying bacteria "decide" or "try" to mutate so they can survive treatment are penalised. Mutation is a random, undirected copying event — bacteria cannot sense an antibiotic and alter their DNA in response. Only what happens after the mutation, the selective killing of non-resistant bacteria, is non-random.
Misconception that bacteria mutate deliberately to gain resistance: w22 Paper 41 Q4b i.
Resistance is not instant or universal
A whole bacterial population does not turn resistant the first time an antibiotic is used. At first only the few bacteria that already carried the mutation survive; the resistant proportion then rises gradually across generations as those survivors reproduce while non-resistant bacteria are killed.
Antibiotic ≠ antibody
These two are repeatedly swapped. An antibiotic is a chemical drug, taken into the body, that kills or inhibits bacteria. An antibody is a protein made by the body's own lymphocytes in response to a specific antigen. Read which word the question uses before choosing an MCQ option or starting a definition.
Antibiotics and antibodies repeatedly confused: s22 Paper 42 Q4b iv.
Match the command word
"State what antibiotics treat" = bacterial infections, one line. "Explain why they do not work on viruses" = name the bacteria-specific target and say the virus lacks it. "Suggest" = apply the resistance mechanism to the exact scenario in the stem.
Use the four-link resistance chain
Random pre-existing mutation → antibiotic kills non-resistant bacteria → resistant bacteria survive and reproduce → resistant proportion increases. Resistance questions are marked against these four links, so "bacteria become immune" collapses all four into nothing.
Answer both halves
Antibiotic questions almost always want two things: what antibiotics do act on, and what they do not. Giving only "bacterial infections", or only "no effect on viruses", forfeits a mark that was there for the taking.
Say why misuse still applies pressure
For "why not prescribe as a precaution", do not stop at "it will not help". Add that the antibiotic still reaches the patient's own bacteria, applying a selection pressure with no medical benefit — that link is where the mark sits.
Drug: any substance taken into the body that modifies or affects chemical reactions in the body. This broad definition covers prescribed medicines, over-the-counter medicines, and recreational or misused substances — legal or illegal.
Antibiotic: a substance (e.g. penicillin, produced by the fungus Penicillium) that kills or inhibits the growth of bacteria. Antibiotics are effective against bacterial infections but have NO effect on viruses.
Antibiotic resistance: the ability of a bacterium to survive exposure to an antibiotic that would normally kill it or stop it growing, arising from a random mutation and spread through a population by natural selection.
Define the term drug.
(a) Define the term drug. [2]
(b) State what type of infection antibiotics, such as penicillin, are used to treat. [1]
(c) State whether antibiotics are effective against viral infections. [1]
