People who begin taking drugs often do so to achieve a certain effect that they find enjoyable or positive in some way. Prescription medications may be taken initially to treat pain, depression, or anxiety. Improper use of prescription and other drugs (including alcohol) may make a person feel alert, powerful, confident, relaxed, friendly, sexy, or talkative. These rewarding consequences increase the likelihood that a person will continue using a drug. Furthermore, two other important consequences that influence continued drug use are tolerance and physical dependence.

Physical dependence occurs when a person's body becomes accustomed to and dependent on the presence of a particular drug. When the dose is lowered or the drug is stopped, the person will begin to notice withdrawal symptoms. Resuming use of the drug eliminates the withdrawal symptoms. Some withdrawal symptoms feel like a flu bug. The individual may...