COCAINE is one of the most powerfully addictive of the drugs of abuse -- and it is a drug that can kill. The pure chemical, cocaine hydrochloride, has been an abused substance for more than 100 years, and coca leaves, the source of cocaine, have been ingested for thousands of years. Cocaine is a strong central nervous system stimulant that interferes with the re-absorption process of dopamine, a chemical messenger associated with pleasure and movement. Dopamine is released as part of the brain's reward system and is involved in the high that characterizes cocaine consumption. The cocaine reaches the brain within seconds, its effects appear almost immediately after a single dose, and disappear within a few minutes or hours. Taken in small amounts (up to 100 mg), cocaine usually makes the user feel euphoric, energetic, talkative, and mentally alert, especially to the sensations of sight, sound, and touch. It can also temporarily decrease the need for food and sleep. Cocaine users frequently find that they need more and more cocaine more often to generate the same level of stimulation. Therefore, any use can lead to addiction. Once having tried cocaine, an individual may have difficulty predicting or controlling the extent to which he or she will continue to use the drug, in another word, no individual can predict whether he or she will become addicted or whether the next dose of cocaine will prove fatal.

Heroin is an illegal, highly addictive drug. It is both the most abused and the most rapidly acting of the opiates. Heroin is processed from morphine, a naturally occurring substance extracted from the seed pod of the Asian poppy plant. Heroin usually appears as a white or brown powder. Heroin is usually injected, sniffed/snorted, or smoked. Typically, a heroin abuser may inject up to four times a day. Intravenous injection provides the greatest intensity and most rapid onset of euphoria (7 to 8 seconds), while intramuscular injection produces a relatively slow onset of euphoria (5 to 8 minutes). When heroin is sniffed or smoked, peak effects are usually felt within 10 to 15 minutes. Although smoking and sniffing heroin do not produce a "rush" as quickly or as intensely as intravenous injection, NIDA researchers have confirmed that all three forms of heroin administration are addictive.  With regular heroin use, tolerance develops. This means the abuser must use more heroin to achieve the same intensity or effect. As higher doses are used over time, physical dependence and addiction develop. With physical dependence, the body has adapted to the presence of the drug and withdrawal symptoms may occur if use is reduced or stopped. Soon after injection (or inhalation), heroin crosses the blood-brain barrier. In the brain, heroin is converted to morphine and binds rapidly to opioid receptors. Heroin is particularly addictive because it enters the brain so rapidly. After the initial effects, abusers usually will be drowsy for several hours. Mental function is clouded by heroin's effect on the central nervous system. Cardiac functions slow. Breathing is also severely slowed, sometimes to the point of death. Heroin overdose is a particular risk on the street, where the amount and purity of the drug cannot be accurately known. One of the most detrimental long-term effects of heroin is addiction itself.  Heroin also produces profound degrees of tolerance and physical dependence, which are also powerful motivating factors for compulsive use and abuse.

Methamphetamine is an addictive stimulant drug that strongly activates certain systems in the brain. It is closely related chemically to amphetamine, but the central nervous system effects of methamphetamine are greater. Both drugs have some medical uses, primarily in the treatment of obesity, but their therapeutic use is limited. Methamphetamine is made in illegal laboratories and has a high potential for abuse and dependence. Street methamphetamine is referred to by many names, such as "speed," "meth," and "chalk." Methamphetamine hydrochloride, clear chunky crystals resembling ice, which can be inhaled by smoking, is referred to as "ice," "crystal," and "glass."

Alcohol Addiction, or dependence, is defined as having at least 3 of the following signs: a tolerance for alcohol (needing increased amounts to achieve the same effect), withdrawal symptoms, taking alcohol in larger amounts that was intended or over a longer period of time than was intended, having a persistent desire to decrease or the inability to decrease the amount of alcohol consumed, spending a great deal of time attempting to acquire alcohol, and finally, continuing to use alcohol even though the person knows there are reoccurring physical or psychological problems being caused by the alcohol. Alcohol is a sedative-hypnotic drug that acts on the human brain like other sedative-hypnotic drugs such as the barbiturates and benzodiazepine tranquilizers. Like other sedative-hypnotic drugs in its class, alcohol can cause physical dependence in anyone who consumes enough of it for a sufficient period of time. The withdrawal syndrome from ethyl alcohol is identical to that for other drugs in the same class such as Valium, Librium, Xanax, Ativan, Phenobarbital and other barbiturates.

Nicotine is an addictive drug. It causes changes in the brain that make people want to use it more and more. In addition, addictive drugs cause unpleasant withdrawal symptoms. The good feelings that result when an addictive drug is present — and the bad feelings when it's absent — make breaking any addiction very difficult. Nicotine addiction has historically been one of the hardest addictions to break. When a person smokes a cigarette, the body responds immediately to the chemical nicotine in the smoke. Nicotine causes a short-term increase in blood pressure, heart rate and the flow of blood from the heart. It also causes the arteries to narrow. The smoke includes carbon monoxide, which reduces the amount of oxygen the blood can carry. This, combined with the nicotine effects, creates an imbalance between the demand for oxygen by the cells and the amount of oxygen the blood can supply. Several symptoms of nicotine withdrawal are listed as follows: irritability; impatience; hostility; anxiety; depressed mood; difficulty concentrating; restlessness; decreased heart rate; increased appetite or weight gain.