What is a drug?
A drug is a substance that modifies chemical reactions in the body. Drugs can change how cells, tissues, organs, or organ systems work. Some drugs are used as medicines to prevent or treat disease; other drugs may be harmful or addictive.
The effect of a drug depends on the chemical reaction or process that it changes. A drug may:
- reduce symptoms;
- destroy or inhibit a pathogen; or
- change the activity of body cells.
This guide focuses on antibiotics and antibiotic resistance.
Antibiotics
Antibiotics are drugs used to treat bacterial infections. They work against bacteria by killing bacterial cells or stopping them from reproducing. This reduces the number of bacteria causing disease and allows the body to recover.
Antibiotics treat bacterial infections; they do not treat viral infections.
Why antibiotics do not affect viruses
Viruses are not bacterial cells. They reproduce inside host cells and do not have the bacterial structures or processes targeted by antibiotics. Therefore, an antibiotic that acts on bacteria does not destroy a virus or stop viral reproduction.
For example, antibiotics should not be used to treat a cold caused by a virus. Using an antibiotic for a viral infection exposes bacteria in the body to the drug without providing a benefit.
Antibiotic resistance
Some bacteria are resistant to particular antibiotics. A resistant bacterium is not killed or inhibited by that antibiotic, so it survives treatment.
When an antibiotic is used:
- Most non-resistant bacteria are killed or stop reproducing.
- Resistant bacteria survive.
- The resistant bacteria reproduce and become a larger proportion of the population.
- The infection becomes harder to treat with that antibiotic.
This is an example of natural selection. The antibiotic does not deliberately make each bacterium resistant. Instead, resistant bacteria that may already be present before treatment survive and reproduce more successfully when susceptible bacteria are removed.
MRSA is an example of antibiotic-resistant bacteria. Infections caused by MRSA can be difficult to treat because some commonly used antibiotics are ineffective against it.
Limiting resistance
Using antibiotics only when essential helps limit the development and spread of resistant bacteria such as MRSA.
Good practice includes:
- using antibiotics for bacterial infections when they are needed;
- not using antibiotics for viral infections;
- following medical instructions about the antibiotic; and
- avoiding unnecessary use of antibiotics in individuals and communities.
Reducing unnecessary exposure means fewer bacteria are placed under selection pressure. Fewer resistant bacteria are therefore selected to survive and reproduce. This helps preserve antibiotic effectiveness for people who need treatment.
Exam summary
- A drug modifies chemical reactions in the body.
- Antibiotics are used to treat bacterial infections.
- Some bacteria are resistant to antibiotics.
- Antibiotics do not affect viruses.
- Unnecessary antibiotic use selects for resistant bacteria.
- Using antibiotics only when essential helps limit resistant bacteria, including MRSA.
Common exam mistake
Do not write that antibiotics cure all infections. State whether the infection is bacterial or viral. Antibiotics act against bacteria, not viruses.