Heroin & Addiction IGCSE Biology 0610

Heroin and Addiction: Effects on the Nervous System

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

Key concept

Heroin is a highly addictive depressant drug derived from morphine. It slows brain activity and blocks pain signals, but repeated use leads to tolerance, physical dependence and severe withdrawal symptoms.

Heroin questions at the Supplement level test understanding of how a drug creates dependence and the difference between physical and psychological addiction. Answers should reference the nervous system rather than just listing social harms.

What heroin does in the body

Heroin is an opiate derived from morphine. It acts as a powerful depressant, meaning it slows the activity of the central nervous system. It binds to receptor molecules on nerve cells in the brain, blocking pain signals and producing intense feelings of relaxation and euphoria.

Because heroin mimics natural pain-relieving chemicals (endorphins), the brain reduces its own production of these chemicals. This change in brain chemistry is central to why addiction develops so rapidly.

Tolerance and physical dependence

With repeated use, the body adapts to the presence of heroin. The user needs increasingly larger doses to achieve the same effect; this is tolerance. At the same time, the body becomes unable to function normally without the drug, leading to physical dependence.

If the drug is withdrawn, the user experiences severe physical symptoms including muscle cramps, vomiting, sweating and anxiety. These withdrawal symptoms drive continued use even when the user wants to stop, which is the hallmark of addiction.

Health and social consequences

Heroin depresses breathing rate, and overdose can cause death by respiratory failure. Sharing needles for injection increases the risk of transmitting blood-borne infections such as HIV and hepatitis. Long-term use damages the liver and kidneys.

Social consequences include loss of employment, breakdown of relationships, criminal activity to fund the addiction, and financial hardship. Exam answers should link biological effects (nervous system depression, tolerance) to these wider consequences rather than listing them in isolation.

Try a focused question (4 marks)

Explain why a person who regularly uses heroin finds it difficult to stop.

Show the answer and marking guidance

Heroin causes physical dependence so the body cannot function normally without it.
Stopping produces painful withdrawal symptoms such as cramps, sweating and nausea.
Tolerance develops so the user needs larger doses for the same effect.
The brain reduces its own production of natural pain-relieving chemicals, making the user reliant on the drug.

Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.

Common mix-ups to avoid

Check these points against your own answer before moving on.

Confusing depressant with sedative or saying heroin speeds up the nervous system.
Describing only social effects without explaining the biological mechanism of dependence.
Using the term addiction without explaining tolerance or withdrawal.