Describe a drug by its effect on body chemistry
Use a biological definition that is broader than illegal substances.
Section 15.1 · Drugs
Exam scope and assessment notes
Core and Extended · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Distinguish bacteria from viruses; recall enzymes, chemical reactions, pathogens and reproduction of bacteria.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which pair belongs in the biological definition?
What you will learn
- Include entry into the body and a chemical effect
- Distinguish a drug from a device or an internal signal
Describe a drug by its effect on body chemistry
| Feature | Meaning |
|---|---|
| Entry | Substance introduced into the body |
| Effect | Changes or influences chemical reactions |
| Scope | Can include medicines; not limited to illegal substances |
| Context | Internal hormone production differs from an introduced preparation |
Include entry into the body and a chemical effect
In this biology section, a drug is a substance introduced into the body that changes or influences chemical reactions within it. The definition includes two links: the substance is taken into the body, and it affects body chemistry. The word drug does not mean only an illegal substance, and a substance need not cause addiction to fit this biological account.
Medicines can be drugs because their chemical effects are used in treatment. For example, a supplied model substance may inhibit a reaction involved in producing a pain signal. A medical device such as an external thermometer measures a condition; the device itself is not a substance introduced to alter body reactions. Classify the role described in the question rather than judging an object only by where it is found.
Distinguish a drug from a device or an internal signal
A drug can increase, decrease or otherwise alter a particular chemical process. It does not have to stop every reaction in the body. Insulin made naturally by the pancreas is an internal hormone signal; a supplied insulin preparation introduced for treatment can also act as a drug. The same named chemical can have different source contexts, so distinguish normal production inside the body from a substance given from outside.
A definition identifies a category, not whether taking a particular substance is safe or medically appropriate. These lessons use invented reaction and pathogen models to practise biological reasoning; they do not ask learners to choose a medicine or a dose. Include the body-entry and chemical-effect links when defining a drug, then use the specific evidence supplied for its effect.
Pause and explain
Can a prescribed medicine fit the definition of a drug?
Worked example
A supplied substance enters the body and slows reaction X. Why does it fit the biological definition?
Show the worked solution
- The substance is introduced into the body.
- It changes a chemical reaction there.
Answer It is a drug in this biological model because it enters the body and affects a chemical reaction.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Define a drug in biological terms.
Give me a hint
Include both the introduced substance and its chemical effect.
Compare my reasoning
- It is a substance taken into the body.
- It changes or influences chemical reactions within the body.
A drug is a substance introduced into the body that changes or influences chemical reactions within it.
Look for these in your work
- Include entry into the body
- Include an effect on chemical reactions
Why is an external thermometer not itself a drug in this definition?
Give me a hint
Its role is measurement rather than an introduced chemical effect.
Compare my reasoning
- It is a measuring device.
- The device is not a substance introduced to alter body reactions.
An external thermometer is a measuring device, not a substance introduced into the body to affect chemical reactions.
Look for these in your work
- Identify the device role
- Apply both definition links
Distinguish insulin produced naturally in the pancreas from an insulin preparation introduced in a supplied treatment model.
Give me a hint
The chemical name is the same but the source context differs.
Compare my reasoning
- Natural production supplies an internal hormone signal.
- The introduced preparation can act as a drug affecting body chemistry.
Naturally produced insulin is an internal hormone signal; an introduced insulin preparation can also act as a drug because it is given from outside and affects body chemistry.
Look for these in your work
- Distinguish the two sources
- Explain the introduced substance role
Common mistakes
- Defining drugs only as illegal substances.
- Saying every drug stops all body chemistry.
What can you explain now?
Can a prescribed medicine fit the definition of a drug?
Compare with the explanation
Yes, if introduced into the body to affect chemical reactions
Legal medicines can fit the biological definition.
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Make it stick
Define a drug using both required links and distinguish a chemical treatment from a measuring device.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.