Crack is such a strong and addictive drug that a person can become addicted the first time they use crack. When a person tries to quit using crack, they go through a process called withdrawal that is very hard to go through. Withdrawal happens when as person's body or mind is used to having a drug in its system all the time, and suddenly the drug is not there any more. Because crack can cause both physical addiction (when the body is hooked on the drug) and psychological addiction (when the mind is hooked on the drug), quitting crack can cause both physical withdrawal (which causes physical symptoms in the body) and psychological withdrawal (which causes symptoms in the mind). Symptoms of (or problems caused by) physical withdrawal can include nausea (feeling like throwing up), vomiting (throwing up), and pain in the muscles. Psychological withdrawal can make a person feel very depressed, anxious (worried and not able to relax), agitated (restless and "keyed up"), or tired. When a person is withdrawing from crack, the urge to use crack will be very, very strong, and it is very hard for the person to fight off these urges and keep away from using the drug again.