People can become dependent on a drug for a number of reasons. For some people it may be because their body gets used to having the drug and when the drug levels in the body drop the person may experience withdrawal symptoms as the body tries to cope and adapt. This is sometimes called physiological dependence. For others it may be a psychological (mental) dependence, or even a combination of both.

A psychological dependence on a drug can happen with all types of drugs, not just those that the body adapts to having and people can become physiologically dependent on. If a person is psychological dependent on a drug they may find that they feel they need the drug to cope with a particular situation. For example, a person may feel they can't have a good night out without drinking alcohol or taking ecstasy.