Enjoyment triggers areas in your brain known as 'pleasure centres'. They release 'feel-good' chemicals, in particular dopamine. All animals have this reward system, usually triggered by food or sex. However, the system can be affected by drugs, including nicotine and alcohol. At first, these act in the same way as 'natural' rewards, producing pleasure. But with increased use, the drug is needed to stop unpleasant symptoms that appear when it is not available ('withdrawal'). These effects contribute to drug addiction.