A use tells you why a drug is given. A mechanism tells you how the drug produces its effect inside the body. Exam questions ask for one or the other, and giving the wrong one is a quiet way to lose marks.
This skill opens drugs and biological effects because every later lesson depends on reading the question word correctly.
What is the difference between a drug, a medicine and a mechanism?
A drug is a substance that changes how the body works. A medicine is a drug used to treat or prevent a health problem. Caffeine and alcohol are drugs; an antibiotic prescribed for a bacterial infection is a medicine.
A mechanism is the chain of steps between the substance entering the body and the effect you can observe. A good mechanism sentence has three parts: what it acts on, what changes, and what that leads to.
How to sort a sentence
Ask two questions of any sentence about a drug.
- Does it say why someone would take it? That is a use.
- Does it say what the substance acts on and what changes? That is a mechanism.
| Question wording | What to write |
|---|---|
| What is it used for? | The condition or purpose |
| How does it work? | Target, change, consequence |
| Why does it not work on a virus? | A target mismatch |
| Suggest an effect on the body | A result linked to a mechanism |
Worked example
Antibiotic A is an invented drug used in this lesson. Read the paragraph and label each sentence.
(a) Antibiotic A is given to treat a chest infection caused by bacteria. (b) It damages the bacterial cells so that they die. (c) It has no effect on viruses.
Sentence (a) says why the drug is given, so it is a use.
Sentence (b) says what the drug acts on (bacterial cells), what changes (they are damaged) and the result (they die). That is a mechanism.
Sentence (c) explains a limit. Viruses are not bacterial cells, so there is no bacterial cell for the drug to damage. That is a target mismatch, which is part of the mechanism idea.
Now answer: “Explain how antibiotic A reduces the infection.”
Model answer: the antibiotic damages the bacterial cells, so the bacteria die and the infection becomes smaller.
The mistake to watch for
Mistaken answer: “Antibiotic A reduces the infection because it treats the infection.”
This repeats the question as an answer, so it is circular. “Treats” is a use, and the question asked for a mechanism.
The correction is to look for the verb that shows a change in the body: damages, blocks, binds, slows, kills. If none of those appears in your sentence, you have probably written a use.
Check yourself
1. Label each statement as a use or a mechanism. (a) “Drug B is taken to lower pain after an operation.” (b) “Drug B binds to receptors on nerve cells so fewer pain messages pass on.”
Show answer
(a) is a use: it says why the drug is taken. (b) is a mechanism: it names the target (receptors on nerve cells), the change (binding) and the result (fewer pain messages).
2. A student writes: “Drug C works by being a medicine for asthma.” Why does this not answer “How does Drug C work?”
Show answer
It names a purpose only and gives no target, change or result. A better answer would say what the drug acts on and what changes as a result.
3. Is alcohol a drug, a medicine, or both? Give a reason.
Show answer
Alcohol is a drug because it changes how the body works, for example by slowing reactions. It is not normally described as a medicine, because it is not used to treat or prevent a health condition.
Where this leads next
With use and mechanism separated, go to interpreting evidence about a stimulant to see how data supports a mechanism. Then try the drugs and biological effects practice set.
Some students can recite definitions but still blur use and mechanism when the wording changes. That is the kind of pattern our teachers look for in online one-to-one Biology tuition.