The person is highly likely to start drinking or using again if they don't face these issues. A young person often becomes attached to the social aspects of their drug use. Drug use can provide meaning for young people who believe that life is otherwise meaningless. Giving up or stopping can leave a perceived void that takes time, and requires alternatives, to fill. Psychological withdrawal can go on for months or years.
People often go back to drug use.
This is not necessarily a failure. The young person may have achieved some short term goals. The terms detoxification and withdrawal are often used interchangeably. It literally means 'un-poisoning' and refers to the process of eliminating the drug from the body. There are a range of options to help detoxification.