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 useful medicines; others are misused and can damage health.
Key idea
Antibiotics (such as penicillin) kill or stop the growth of bacteria. They do not work against viruses, because viruses have a different structure and reproduce inside our cells.
Antibiotic resistance
If antibiotics are overused or a course is not finished, some bacteria survive and become resistant. These multiply, so the antibiotic no longer works. To reduce resistance, doctors avoid unnecessary antibiotics and patients complete the full course.
Remember
- Take antibiotics only when needed and always finish the course.
Misused drugs
Some drugs harm the body when misused:
- Alcohol — a depressant; slows reactions, damages the liver (cirrhosis), affects judgement.
- Tobacco smoke — contains nicotine (addictive), tar (a carcinogen linked to lung cancer) and carbon monoxide (reduces oxygen carried by blood).
- Heroin — a strongly addictive depressant.
Many misused drugs cause addiction, meaning the user feels a strong need to keep taking the drug and suffers unpleasant withdrawal symptoms when they stop. Misused drugs may also harm other people, for example when alcohol slows a driver's reactions and causes accidents.
Worked example
A smoker's blood carries less oxygen. Carbon monoxide binds to haemoglobin in place of oxygen, so fewer oxygen molecules are transported. During exercise the smoker tires quickly because muscles cannot get enough oxygen for aerobic respiration, and over many years the tar in the smoke can lead to lung cancer.