Drug addiction is compulsive use of a substance despite negative consequences which can be severe; drug abuse is simply excessive use of a drug or use of a drug for purposes for which it was not medically intended. Dependence on a substance is not necessary or sufficient to define addiction, there are some substances that don't cause addiction but do cause dependence (for example, some blood pressure medications) and substances that cause addiction but not dependence (they are mainly characterized by depression).

Causes, incidence, and risk factors
Drug abuse can lead to drug dependence or addiction. Drug addiction may also follow the use of drugs for physical pain relief, though this is rare in people without a previous history of addiction. The exact reason of drug abuse and dependence is not yet known. The genetic make-up of the individuals, peer pressure, emotional distress, anxiety, depression, and environmental stress are all factors which seem to be involved. Children who grow up in an environment of illicit drug use may first see their elders using drugs. This may put them at a higher risk for developing an addiction later in life for both environmental and genetic reasons.

Commonly abused substances include:
-  Opiates and narcotics are powerful painkillers with sedative and euphoric qualities. These include heroin, opium, codeine, Oxycontin and others.
-  Central nervous system stimulants have a stimulating effect and can produce tolerance. These include amphetamines, cocaine, commonly used stimulants are caffeine and nicotine.
-  Central nervous system depressants produce a soothing sedative and anxiety-reducing effect and which leads to dependence. These include barbiturates (amobarbital, pentobarbital, secobarbital). Commonly used depressants, by far, is alcohol.
-  Hallucinogens produce psychological dependence. These include LSD, mescaline, psilocybin ('mushrooms').
-  Tetrahydrocannabinol (THC) is the active ingredient found in cannabis, marijuana, and hashish. Although used for their relaxing properties, THC-derived drugs can also lead to paranoia and anxiety.

Drug intoxication and drug overdose may be accidental or intentional. Drug withdrawal symptoms can occur when use of a substance is stopped. Withdrawal symptoms vary, depending on the particular substance. The withdrawal symptom depends on the length of time the drug was being used. Drug intoxication, overdose, and withdrawal can be life-threatening in some situations.

Treatment for the person with drug addiction begins with the recognition of the problem. Though earlier 'denial' was considered as a symptom of addiction, recent studies has shown that this symptom can be dramatically controlled if addicts are treated with love and care, rather than being told what to do or 'confronted.' Treatment of drug addiction involves detoxification, support and abstinence. Emergency treatment may be indicated for acute cases. Often, there may be a loss of consciousness and the person may need special medical attention temporarily. The specific treatment depends on the drug. Detoxification is the gradual withdrawal of an abused substance in a controlled way. Sometimes a drug with a similar action is substituted during the withdrawal process to reduce the unpleasant symptoms and risks associated with withdrawal. If depression or other mood disorder exists, it should be treated appropriately. The need for treatment is underscored by the severity of illness of those who undergo detoxification and the societal costs of untreated substance use disorders.

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