Drugs Notes

Cambridge IGCSE Biology: Revision notes

Key facts

  • A drug is a substance taken into the body that modifies or affects chemical reactions in the body.
  • Antibiotics kill or inhibit bacteria; they do not work against viruses.
  • Resistance develops by natural selection: resistant bacteria survive, reproduce and spread, as with MRSA.
  • To limit resistance, antibiotics should be used only when essential, and the full course finished.

What a drug is

A drug is a substance taken into the body that modifies or affects chemical reactions in the body.

Use the precise definition. 'Medicine' or 'treatment' is too vague, and a drug is not just something a doctor prescribes.

  1. 1

    Taken into the body

    swallowed, injected, inhaled or absorbed

  2. 2

    Modifies chemical reactions

    in the body

  3. 3

    Effect

    for example treating disease, relieving pain or changing behaviour

What a drug does.
  • Drugmodifies chemical reactions in the body

Which best defines a drug?

Antibiotics

Antibiotics kill bacteria or stop them growing, but have no effect on viruses.

Antibiotics treat bacterial infections only. Viruses lack the structures antibiotics target, such as bacterial cell walls, so antibiotics do not work against colds or flu. Never say antibiotics kill viruses.

  1. 1

    Antibiotic

    targets bacterial structures such as the cell wall

  2. 2

    Bacterial infection

    bacteria are killed or stop growing

  3. 3

    Viral infection

    viruses lack those structures, so there is no effect

Why antibiotics work on bacteria but not viruses.

Caused by bacteria

Examples:
Pneumonia, urinary tract infection, skin infection
Antibiotics work?:
Yes

Caused by viruses

Examples:
Cold, flu
Antibiotics work?:
No

Why are antibiotics ineffective against flu?

Resistance

Resistant bacteria survive treatment, reproduce and pass on resistance, so the population becomes resistant.

This is natural selection. Bacteria vary, and some have genes that make them resistant. An antibiotic kills the susceptible bacteria, the resistant ones survive and reproduce, and resistant strains come to dominate. MRSA is an antibiotic-resistant bacterium that causes serious infections.

  1. 1

    Variation

    Some bacteria have resistance genes

  2. 2

    Antibiotic used

    Susceptible bacteria are killed

  3. 3

    Resistant survive

    They reproduce

  4. 4

    Resistant population

    Resistant strains dominate

How resistance develops

What is MRSA?

Limiting resistance

Use antibiotics only when they are essential, and finish the course.

If a patient gets antibiotics for a viral cold, bacteria in their body are exposed to the drug: the susceptible ones die and the resistant ones survive, spreading resistance. Refusing to prescribe prevents this. Finishing a course makes sure all the bacteria are killed, so fewer resistant ones survive.

  1. 1

    Antibiotic taken

    for example for a viral cold, or a course stopped early

  2. 2

    Susceptible bacteria die

    resistant ones survive

  3. 3

    Resistant bacteria reproduce

    passing on the resistance

  4. 4

    Resistance spreads

    so the antibiotic works less well

How misuse of antibiotics spreads resistance.

Limits resistance

Prescribing:
Only when essential
Taking the course:
Finish all the tablets

Increases resistance

Prescribing:
For viral infections
Taking the course:
Stop early when feeling better

Why should a patient finish a full course of antibiotics?

Try an exam question

Explain how overuse of antibiotics can lead to the spread of antibiotic-resistant bacteria.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.