Withdrawal symptoms are one reason that the success rate for rehabilitation is so low. One leading psychological withdrawal symptom is a burning desire for more of the drug. This symptom is more difficult to dismiss than any of the others because it persists and wears the person down in a way that physical, tangible pain does not.
Most drugs should not be quit "cold turkey." It is best to gradually take less of the drug until it is deemed physically and psychologically safe to stop entirely. Withdrawal can cause symptoms such as chills, cold sweats, hot flashes, nausea, dizziness, vomiting, seizures, or even hallucinations. Some kinds of withdrawal are very dangerous because withdrawal from certain drugs can lead to brain trauma if not done properly.
Detoxification is also potentially dangerous if not done with professional care. Some drugs cause physiological addiction; others cause psychological addiction. The most powerful substances, in terms of encouraging abuse, cause both.