When your body is exposed to any type of substance for a regular period of time, the substance throws off your body’s natural balance or homeostasis.
When you regularly take a substance for a period of time, your body may build a tolerance and dependence on that substance. Tolerance means that it takes larger doses of the substance to achieve the same effects that you initially experienced, while dependence means that your body requires the substance in order to avoid experiencing withdrawal effects.
If you abruptly stop or decrease your intake of the substance, your body is once again thrown off balance and symptoms of withdrawal may result. Such symptoms are often both physical and mental, and can potentially be dangerous depending on the type of drug.
Withdrawal symptoms are often the opposite of the effects of the substance. For example, alcohol is a depressant, so if you suddenly stop consuming alcohol, you might experience symptoms of overstimulation such as anxiety or restlessness.