Heroin is an illegal, very addictive, opiate type drug. Heroin use is more widespread than any other opiate. Heroin is processed from morphine, a naturally occurring substance extracted from the seed pod of certain varieties of poppy plants. Heroin is usually sold as a white or brownish powder, or as the black sticky substance known on the streets as "black tar heroin." Although purer heroin has become more common recently, most heroin sold on the street is "cut" with other drugs or with substances such as sugar, starch, powdered milk, or quinine. Some street variety heroin may also be cut with strychnine or other toxic chemicals. Because heroin addicts do not know the actual strength of the heroin they obtain or its true contents, they are constantly at risk of overdose or death. Heroin addiction also poses others dangers because of the possiblity of transmission of HIV and other life threatening diseases that can occur from sharing needles or other injection equipment.

One of the most detrimental long-term effects of heroin is heroin addiction itself. Heroin addiction is a chronic problem characterized by compulsive heroin seeking and heroin use, and by neurochemical and molecular changes in the brain. Heroin also produces a profound degree of tolerance and physical dependence to the drug, which are powerful motivating factors for compulsive use resulting in addiction. As with regular use of any addictive opiate, heroin addicts gradually begin to spend more time and energy obtaining and using heroin to get high and to avoid withdrawal symptoms. Once addiction occurs, the heroin addict's primary purpose in life becomes seeking and using heroin.

Physical dependence develops with higher doses of heroin. With physical dependence, the body adapts to the presence of the drug and withdrawal symptoms occur if the use of heroin is reduced abruptly. Withdrawal symptoms may occur within a few hours after the last time that heroin is taken. Symptoms of heroin withdrawal include restlessness, muscle and bone pain, insomnia, diarrhea, vomiting, cold flashes with goose bumps ("cold turkey"), and leg movements. Major heroin withdrawal symptoms peak between 24 and 48 hours after the last dose of heroin and subside after about a week.

Heroin addiction like all opiate addictions occurs when heroin is used over a sustained period of time. The onset of heroin addiction can be both rapid and severe, dependending upon the amount of heroin used and frequency in a designated period of time. Heroin addicts will "crave" more of the drug and experience withdrawal symptoms if they do not get their regular "fix" or dose. Not all of the mechanisms by which heroin and other opiates affect the brain are known. Likewise, the exact brain mechanisms that cause tolerance and addiction are not completely understood. Heroin stimulates a "pleasure system" in the brain. This system involves neurons in the mid-brain that use the neurotransmitter called "dopamine." These mid-brain dopamine neurons project to another structure called the nucleus accumbens which then projects to the cerebral cortex. This system is responsible for the pleasurable effects of heroin and for the addictive power of heroin.

Heroin withdrawal begins with a three- to five-day period of intense anxiety, INSOMNIA, and a host of flu-like symptoms from uncontrolled coughing and yawning to stuffy nose, cramps, chills, sweating, diarrhea, and "goose bumps." Having goose bumps led to the origin of the phrase "quitting cold turkey." Additionally, muscles that have been relaxed by the drug tighten and twitch, causing severe pain and uncontrolled, reflexive motion ("kicking the habit").