Aversion therapy is a behavioural therapy used in the reduction of addictive behaviour. Aversion therapy is based on classical conditioning and involves the person creating a learned association between an aversive stimulus (something unpleasant) and their addictive behaviour (e.g. smoking, gambling, taking drugs etc.). For example, if someone was being treated for a smoking addiction they would be asked to smoke (neutral stimulus), but at the same time be exposed to a drug (unconditioned stimulus) to make them feel sick (unconditioned response). After a number of pairings, smoking a cigarette will become a conditioned stimulus, and the person will feel sick (conditioned response), even though they have not been given the drug. This should reduce their addictive behaviour.