Overview
The sustained use of many kinds of drugs causes adaptations within the body that tend to lessen the drug's original effects over time, a phenomenon known as drug tolerance. At this point, one is said to also have a physical dependency on the given chemical. This is the stage that withdrawal may be experienced upon discontinuation. Some of these symptoms are generally the opposite of the drug's direct effect on the body. Depending on the length of time a drug takes to leave the bloodstream elimination half-life, withdrawal symptoms can appear within a few hours to several days after discontinuation and may also occur in the form of cravings. A craving is the strong desire to obtain, and use a drug or other substance similar to other cravings one might experience for food and hunger.
Although withdrawal symptoms are often associated with the use of recreational drugs, many drugs have a profound effect on the user when stopped. When withdrawal from any medication occurs it can be harmful or even fatal; hence prescription warning labels explicitly saying not to discontinue the drug without doctor approval.
The symptoms from withdrawal may be even more dramatic when the drug has masked prolonged malnutrition, disease, chronic pain, or sleep deprivation, conditions that drug abusers often suffer as a secondary consequence of the drug. Many drugs (including alcohol) suppress appetite while simultaneously consuming any money that might have been spent on food. When the drug is removed, the discomforts return in force and are sometimes confused with addiction withdrawal symptoms, which they quite properly are not.
Read more about this topic: Withdrawal