Revision Notes

15.1 Medicinal Drugs

What is a Drug?

  • A drug is any substance taken into the body that modifies or affects chemical reactions in the body
  • Some drugs are medicinal drugs that are used to treat the symptoms or causes of a disease – for example, antibiotics


  • Antibiotics are chemical substances made by certain fungi or bacteria that affect the working of bacterial cells, either by disrupting their structure or function or by preventing them from reproducing.
  • Antibiotics are effective against bacteria but not against viruses.
  • Antibiotics target processes and structures that are specific to bacterial (prokaryotic) cells; as such they do not generally harm animal cells.

How antibiotics work, IGCSE & GCSE Biology revision notesHow antibiotics work

Effectiveness of Antibiotics

  • Some bacteria that cause disease have become resistant to antibiotics and this reduces the effectiveness of prescribed antibiotics when someone has a bacterial infection, as it might be caused by a type of bacteria that is resistant to that particular antibiotic
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Why don't Antibiotics Affect Viruses?

  • Viruses cannot be treated with antibiotics
  • This is because antibiotics work by disrupting cell functions such as respiration, or breaking down the structure of the cell in some way
  • However, viruses do not carry out any cell functions and do not have cell walls, cell membranes or any cell organelles as viruses infect and utilise the machinery of animal cells to reproduce, which are not affected by antibiotics.
  • Therefore the action of antibiotics do not affect them
Extended Only

Antibiotic Resistance

  • Since the first antibiotic was discovered in 1928, many more have been discovered and developed and antibiotics were and are widely overused
  • Commonly prescribed antibiotics are becoming less effective due to a number of reasons:
    • overuse and being prescribed when not really necessary
    • patients failing to complete the fully prescribed course by a doctor
    • large scale use of antibiotics in farming to prevent disease when livestock are kept in close quarters, even when animals are not actually sick
  • This has lead to the effectiveness of antibiotics being reduced, and the incidence of antibiotic resistance increasing
  • These bacteria are commonly known as superbugs and the most common is MRSA
  • Ways individuals can help prevent the incidence of antibiotic resistance increasing include:
    • only taking antibiotics when absolutely essential
    • when prescribed a course of antibiotics, ensure that the entire course is completed even if you feel better after a few days

Antibiotic_resistance, IGCSE & GCSE Biology revision notesAntibiotic resistance

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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