With a few drugs, the potential dangers are extremely high and they can cause serious problems no matter how or when they are taken. Dependence refers to a physical state where when one stops using a drug or alcohol, withdrawal occurs. If a person experiences withdrawal symptoms after discontinuing use of drugs or alcohol, then he or she is dependent on that drug or alcohol. After repeated use of a drug the user's body becomes so accustomed to a particular drug that it can only function normally if the drug is present. Often the terms addiction and dependence are used to mean the same thing.
Physical dependence is one of the factors contributing to the continued use of drugs.
Psychological dependence exists when a drug is so central to a person's thoughts, emotions, and activities that it is extremely difficult to stop using it, or even stop thinking about it. Like physical dependence, psychological dependence is a cause of continued drug use. Over time with regular use, a user needs more and more of the drug to get the desired effect. Increased tolerance means an increased risk for overdose for two reasons. Tolerance does not develop evenly to all effects of a drug.