When a person uses addictive substances repeatedly, his or her body develops tolerance and requires great amounts to achieve the same effect. Thus, with increased dosages, the body starts to depend on the drugs to replace the natural chemicals. Over time, both the mind and body of the person get locked into the addictive pattern. As the body craves for addictive substance, if it is missing, the withdrawal symptoms kick in. Thus, an addict may suffer from exhaustion, anxiety, depression, and other similar conditions. The change can be quite quickly if you are using Ecstacy, crack cocaine, methamphetamine, or oxycodone. On the other hand, it can be slow as the case with marijuana and alcohol.