Types of medicinal drugs
Medicinal drugs are chemical substances used to treat, prevent or diagnose disease. They include painkillers that relieve symptoms without curing the disease, and antibiotics that target the pathogen itself. Vaccines are also medicinal, preventing disease by stimulating immunity before infection occurs.
It is important to distinguish between drugs that treat symptoms (such as paracetamol reducing fever) and those that treat the cause (such as antibiotics destroying bacteria). Exam questions may ask students to classify a drug by what it does rather than what it is called.
How antibiotics work against bacteria
Antibiotics are substances produced by microorganisms, or synthesised chemically, that kill or inhibit the growth of bacteria. Penicillin, discovered by Alexander Fleming, works by preventing bacteria from forming proper cell walls. When bacteria divide, the new cells cannot maintain their shape and burst. Other antibiotics interfere with bacterial protein synthesis or DNA replication.
The key point for IGCSE is that antibiotics target structures or processes found in bacteria but not in human cells. This selectivity means they can destroy the pathogen without seriously harming the patient.
Why antibiotics do not work on viruses
Viruses are not cells. They lack cell walls, ribosomes and their own metabolic machinery. Instead, viruses inject their genetic material into a host cell and use the host cell's machinery to replicate. Antibiotics target bacterial structures such as cell walls and bacterial ribosomes, which viruses do not possess.
Therefore, prescribing antibiotics for a viral infection such as influenza or the common cold is ineffective. This is a very frequently tested point: antibiotics kill bacteria but have no effect on viruses.
Responsible use of antibiotics
Patients must complete the full prescribed course of antibiotics even if they feel better. Stopping early leaves some bacteria alive, potentially those with slight natural resistance to the antibiotic. These survivors can multiply and spread, increasing the proportion of resistant bacteria in the population.
Overuse and misuse of antibiotics, including their use in livestock farming, accelerate the development of antibiotic-resistant strains. This is why doctors should only prescribe antibiotics when a bacterial infection is confirmed.