Heroin is an illicit, highly addictive drug. It is either the most abused or the most rapidly acting member of opioids. Abusers describe a feeling of a surge of pleasurable sensation, named as "rush" or "high". Repeated administration of high doses of heroin results in the induction of physical dependence. Physical dependence refers to an altered physiological state produced by chronic administration of heroin which necessitates the continued administration of the drug to prevent the appearance of a characteristic syndrome, the opioid withdrawal or abstinence syndrome. Withdrawal symptoms may occur within a few hours after the last administration of heroin. Symptoms of the withdrawal include restlessness, insomnia, drug craving, diarrhea, muscle and bone pain, cold flashes with goose bumps, and leg movements. Major withdrawal symptoms peak between 48 and 72 hours after the last dose of heroin and subside after about a week. At this time, weakness and depression are pronounced and nausea and vomiting are common. Nevertheless, some chronic addicts have shown persistent withdrawal signs for many months or even years. Heroin addiction is considered as a behavioural state of compulsive drug use and a high tendency to relapse after periods of abstinence. It is generally accepted that compulsive use and relapse are typically associated with the status of heroin craving or heroin hunger that are difficult to define but appear to be powerful motivational significance in the addiction process. The route of administering heroin varies largely and may indicate the degree of seriousness of the individual's addiction. Intravenous administration seems to be the predominant method of heroin use, but recently a shift in heroin use pattern has been found, i.e. from injection to sniffing and smoking. Frequent injections coupled with widespread sharing of syringes increase the risk of contracting HIV, hepatitis B, C and other blood-borne infectious diseases. Long-term use of heroin has also severe medical consequences such as scarred veins, bacterial infections of blood vessels, liver and kidney diseases, and lung complications.