Alcohol, Tobacco and Drugs: KS3 Biology Health Guide

What is a drug and how does it affect the body?

A drug is any substance that alters the normal chemical processes in the body. Medicines are drugs — they are given at controlled doses to treat illness. Recreational drugs are taken for their effects on mood or perception, often with significant health risks.

Drugs are classified by the type of effect they have on the nervous system:

• Stimulants speed up nervous activity — the heart beats faster, alertness increases, reaction times shorten. Examples: caffeine, nicotine, amphetamines.
• Depressants slow down nervous activity — reactions slow, coordination deteriorates, inhibitions reduce. Examples: alcohol, benzodiazepines.
• Hallucinogens distort perception of reality — producing sensations of things that are not real. Examples: cannabis (partly), LSD.
• Painkillers (analgesics) block pain signals. Examples: paracetamol, opioids.

What does alcohol do to the human body?

Alcohol (ethanol) is a depressant that slows the activity of the central nervous system. Its effects depend on how much is consumed:

Long-term effects of regular heavy alcohol consumption:

• Liver cirrhosis: alcohol is broken down in the liver, producing toxic by-products that destroy liver cells and replace them with scar tissue.
• Brain damage: chronic alcohol use shrinks brain tissue, impairing memory and decision-making permanently.
• Heart disease: increases blood pressure and the risk of cardiomyopathy (weakening of the heart muscle).
• Dependence: the brain adapts to the presence of alcohol; stopping suddenly causes severe withdrawal symptoms.

What does tobacco do to the body?

Tobacco smoke contains thousands of chemicals. The main harmful ones are:

Effects on the respiratory system:

• Tar damages the cilia — tiny hair-like structures lining the airways. Normally, cilia sweep mucus and trapped particles upward. When cilia are damaged, mucus accumulates, causing a persistent "smoker's cough".
• Chronic inflammation leads to chronic bronchitis (persistent inflamed airways) and emphysema (damaged alveoli, reduced surface area for gas exchange).
• Carcinogens in tar can cause lung cancer by mutating the DNA of airway cells.

How does addiction develop?

Addiction (physical dependence) develops when the brain adapts to the repeated presence of a drug:

1. The drug alters neurotransmitter levels in the brain's reward pathway.
2. The brain reduces its own production of those neurotransmitters to compensate.
3. Without the drug, neurotransmitter levels are abnormally low — causing withdrawal symptoms (anxiety, shaking, nausea, pain).
4. Taking the drug again relieves withdrawal, reinforcing the behaviour.

Nicotine is highly addictive through this mechanism. Tolerance also develops — the brain requires ever-increasing doses to produce the same effect, which is why it is difficult to quit without support.

How do drugs and alcohol affect reaction times?

Reaction time — the time between a stimulus and a response — is directly affected by depressants and alcohol. This is easy to measure experimentally (e.g. ruler-drop tests). Even small amounts of alcohol significantly slow reaction times, which is why drink-driving laws exist: a driver who has been drinking has slower braking responses and reduced ability to judge speed and distance.

Frequently asked questions

Why is nicotine addictive but not the most harmful chemical in tobacco?

Nicotine causes addiction by triggering the brain's reward system, creating strong cravings and withdrawal symptoms that make it very difficult to stop smoking. However, nicotine itself is not the main cause of cancer or lung disease — the tar and carcinogens in smoke are. This is the basis of nicotine replacement therapy: patches and gums deliver nicotine (to prevent withdrawal) without delivering the harmful smoke chemicals, helping people break the smoking habit gradually.

How does carbon monoxide in cigarette smoke reduce oxygen delivery?

Red blood cells carry oxygen using a protein called haemoglobin. Haemoglobin binds carbon monoxide about 250 times more strongly than it binds oxygen. When carbon monoxide is inhaled, it displaces oxygen from haemoglobin, forming a stable compound called carboxyhaemoglobin. This compound cannot carry oxygen, so fewer red blood cells are available to transport oxygen to tissues. This is why smokers often have lower exercise tolerance and why carbon monoxide poisoning is life-threatening.

Why is the liver particularly vulnerable to alcohol damage?

The liver is the main organ responsible for breaking down (metabolising) alcohol. When alcohol is oxidised in the liver, the intermediate product is acetaldehyde — a toxic compound that directly damages liver cells, causing inflammation. With repeated heavy drinking, inflammation becomes chronic, and the liver replaces damaged cells with fibrous scar tissue (cirrhosis). Scar tissue cannot perform liver functions — detoxification, protein synthesis, bile production — so cirrhosis progressively impairs the entire body's chemical regulation.

Are all drugs illegal?

No. Legal drugs include alcohol, tobacco (nicotine), caffeine and prescription medicines. Illegal drugs are classified under the Misuse of Drugs Act 1971 into Classes A, B and C based on their potential for harm and misuse. Alcohol and tobacco are legal but heavily regulated, and both carry serious health risks — they are responsible for far more deaths annually in the UK than all illegal drugs combined. The legal status of a drug does not determine its safety.

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