Module 6: how drugs work: learner's workbook

3.1 Classifying drugs by their effect on the central nervous system

Page last updated: 2004

Drugs can be classified in many ways. For example, they can be classified according to:

  • uses (medicinal or recreational)
  • effect on the body (the specific effect on the central nervous system)
  • source of the substance (synthetic or plant)
  • legal status (legal/illegal)
  • risk status (dangerous/safe).
One of the most common and useful ways of classifying a drug is by the effect that it has on a person's central nervous system. The brain is the major part of the central nervous system, and this is where psycho-active drugs have their main effect.

The below sub-section summarises the major classifications of drugs including stimulants, depressants and hallucinogens. The group 'others' includes those psycho-active drugs that do not fit neatly in any other category. Some drugs can be classified in a number of categories, e.g. cannabis and ecstasy.

Classifying drugs by their effect on CNS


Tend to speed up the activity of a person's central nervous system (CNS) including the brain.

These drugs often result in the user feeling more alert and more energetic.

Examples include:
  • Amphetamines
  • Cocaine
  • Pseudoephidrine (found in medications such as Sudafed, Codral Cold and Flu)
  • Nicotine
  • CaffeineTop of page

Depressants (also known as relaxants)

Tend to slow down the activity of the CNS, which often results in the user feeling less pain, more relaxed and sleepy.

These symptoms may be noticeable when a drug is taken in large amounts.

It is important to note that the term 'depressant' is used to describe the effect on the CNS, not mood.

CNS depressants are more likely to result in euphoria than depression, especially in moderate use.

Examples include:
  • Alcohol
  • Major tranquillisers
  • Benzodiazepines (e.g. Valium, Temazepam) Opioids (heroin, morphine)
  • Volatile substances (can also be classified as 'other' (glue, petrol, and paint).


Have the ability to alter a user's sensory perceptions by distorting the messages carried in the CNS. A common example is LSD (trips).

Hallucinogens alter one's perceptions and states of consciousness.

Examples include:
  • LSD
  • Psilocybin (magic mushrooms)
  • Mescaline (peyote cactus)


Includes psycho-active drugs that do not fit neatly into one of the other categories, but which are clearly psycho-active, such as antidepressants (e.g. Zoloft) and mood stabilisers (e.g. Lithium).

Examples include:
  • MDMA (ecstasy)*
  • Cannabis*
  • Volatile substances (petrol, glue, paint)

* Both ecstasy and cannabis can produce hallucinations, especially in cases of heavy use, or inexperienced users. However they are usually considered primarily as CNS stimulants and depressants respectively, as these effects are almost always present.