3.6 Harmful substances
Summary: There are some substances that we should not put inside our body. They can damage our health, even in small amounts. These substances include drugs that are not prescribed given to us by a doctor,… Tags: science oxford-science student-book-7 biology Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
- describe what a drug does
- describe the dangers of smoking tobacco
- explain why alcohol is a harmful substance.
Think back
- Name two things you should do to stay healthy.
- Name some medicines people take when they feel unwell.
- Name the organ system that controls body reactions.
Key idea Medicinal drugs (used in medicine) help manage pain or make us better. However, too many medicinal drugs, smoking, and alcohol cause our bodies harm.
Key words drug, medicinal drug, alcohol, ethanol, depressant, tobacco, tar, nicotine, stimulant, carbon monoxide, passive smoking
There are some substances that we should not put inside our body. They can damage our health, even in small amounts. These substances include drugs that are not prescribed (given to us) by a doctor, alcohol, and tobacco in cigarettes.
What are drugs?
Drugs are chemical substances that affect the way your body works. They change the Chemical Reactions that take place. As a result, some drugs can seriously harm you. These drugs are illegal in many countries, which means you are breaking the law if you take them.
However, some drugs can save your life and are used in medicine.
A State the meaning of a drug.
What are medicinal drugs?
Medicinal drugs are used in medicine. They benefit your health in some way. They may be used to treat the symptoms of a condition. For example, paracetamol reduces pain. Other drugs can make you better. For example, the antibiotic pills in Figure 1 can treat chest infections by killing the bacteria that cause the infection.
Figure 1Antibiotic pills treat bacterial infections. A doctor will explain how to use them safely.
However, even medicinal drugs can harm you if you do not take them in the right way. Some medicinal drugs have unwanted side effects. When prescribing drugs, doctors have to consider the benefits of a person taking a drug against any possible risks.
B Explain why a person may take a medicinal drug.
What is alcohol?
Alcohol contains the chemical compound ethanol. When a person drinks alcohol, ethanol enters their blood. It then travels to their brain, where it affects their nervous system. You can read the harmful effects of alcohol in Figure 2.
Figure 2This diagram shows what happens to a person as they increase their alcohol intake. Full text alternative to the content
Why is alcohol dangerous?
Alcohol is a depressant. This means it slows down reactions in the body. Drinking large volumes of alcohol can also cause stomach ulcers, heart disease, and brain and liver damage. This is because the liver breaks down harmful chemicals, including ethanol, into waste Products. Waste products are then removed from the body.
Pregnant women should not drink alcohol because it is harmful to them and their baby. Alcohol diffuses into the baby’s blood through the placenta.
C Describe three harmful side-effects of drinking alcohol.
What is tobacco?
Cigarettes contain tobacco. Tobacco smoke contains:
- tar – a sticky, black substance that builds up in the lungs and narrows the airways
- nicotine – a drug (called a stimulant) that speeds up the nervous system, making the heart beat faster and narrowing blood vessels
- carbon monoxide – a poisonous gas that stops the blood from carrying as much oxygen as it should.
Why is smoking dangerous?
People who smoke will more likely develop conditions such as breathing problems, cancer, heart attacks, and strokes. Breathing in other people’s smoke can also cause harm. This is called passive smoking.
Pregnant women should not smoke. It can harm a developing fetus and will greatly increase the risk of losing a baby.
D Write down the meaning of passive smoking.
Stretch zone
- Copy and complete the sentences. Drugs are chemicals/reactions that affect the way our body works. Many drugs are medicinal/illegal and can harm us. However, medicinal/illegal drugs can help us feel better. For example, antibiotic/antigen drugs can treat a chest infection. Penicillin/Paracetamol reduces pain, but it is only safe in recommended doses.
- Describe one harmful side-effect of smoking on the body.
Stretch zone
- Explain why alcohol is described as a depressant, and why this is harmful.