Module 6: how drugs work: learner's workbook

4.1 The effect of drugs on the central nervous system (CNS)

Page last updated: 2004

Drugs produce their effect on the body through two major processes. The first is the effect of the chemical properties of the drug on the central nervous system (CNS) which includes the brain and the spinal cord. This process is called pharmacodynamics. The second is how the drugs enter, are metabolised, and absorbed by the body. This process is known as pharmacokinetics. These two processes work together to produce a certain effect.

Pharmacodynamic processes


A psycho-active drug must find its way to the bloodstream to have an effect on the brain. Once the drug reaches the brain, it can lodge on to specific receptor sites on the neurons which are sensitive to particular types of drugs. Each drug affects specific neurons in a number of parts of the brain. There are 13 billion neurons or nerve cells in each person's brain.


Many drugs seem to imitate neurotransmitters, the natural chemicals that facilitate or inhibit the transfer of electrical impulses between neurons. For example, opiate drugs such as heroin are thought to exert their drug action by mimicking endorphins which are naturally occurring proteins that reduce pain.

Drug action

Like neurotransmitters, drugs can speed up (CNS stimulants) or slow down (CNS depressants) the transfer of electro-chemical messages between neurons in the brain. Messages between neurons can also be distorted when hallucinogenic drugs are taken.

Pleasure centre

In addition to affecting the transfer of messages between neurons, drugs appear to act directly on 'pleasure centres' in the brain, which may explain the euphoria experienced by users of many different types of drugs. It is believed that the effect on the pleasure centre is highly rewarding for many young people and is crucial to the development of drug dependence.

Review Quiz

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Task - writing exercise

Complete the following quick quiz which reflects your learning so far.
  1. Pharmacology is :

    1. a branch of science that deals with emotions
    2. the study of how drugs work
    3. the study of living things

  2. Circle the drug that is not psycho-active.

    1. alcohol
    2. petrol
    3. antibiotic

  3. A neuron is a:

    1. chemical in the brain
    2. gap between nerve cells
    3. nerve cell

  4. Drugs work by:

    1. imitating neurotransmitters
    2. destroying brain cells
    3. creating dysfunctional neural pathways

  5. The euphoria (good feeling) that drug use promote is caused by:

    1. the distortion of electrochemical messages between neurons
    2. stimulation of pleasure centres in the brain
    3. elimination of withdrawal symptoms

  6. Drugs can be classified by their effect on the CNS. What are the three major groups called? Provide two examples of drugs that fit in each of these categories.

    • Group 1
      • Group Name -
      • Example 1 -
      • Example 2 -

    • Group 2
      • Group name -
      • Example 1 -
      • Example 2 -

    • Group 3
      • Group name -
      • Example 1 -
      • Example 2 -

  7. Why is it necessary to have a fourth group called 'others'?
(Check your answers on the possible answers page.)