For most addicts and alcoholics the cessation of substance use will result in severe physical withdrawal symptoms. Over time, the body becomes habituated to the presence of the substance, and begins to adjust to its effects. This manifests during the addiction as tolerance, the user needs to progressively increase the dose to achieve the desired effect. In essence, the body comes to rely on the presence of this substance to artificially suppress, elevate or both one of its normal functions. When the substance is removed suddenly the body overcompensates drastically, usually resulting in extreme tiredness and depression (in the case of stimulants), or extreme anxiety, panic, cold sweats, and sometimes even seizures (in the case of depressants).
In some cases, detox can actually be a life-threatening proposition. The body can over-compensate so much that it causes a seizure or aneurysm, which can be fatal if the patient is not in the hands of trained professionals. This is why it is important that, at the very least for the detox period, you should be under the supervision of a medical doctor. Some people might believe that they have the willpower to quit on their own, and they might, but even if you intend to detox cold turkey without any detox medications, it is extremely important that a medical professional be monitoring your progress and aware that you are attempting this in case life-threatening complications arise.