Addiction

Historically, addiction has been defined as physical and psychological dependence on psychoactive substances (for example alcohol, tobacco, heroin and other drugs) which cross the blood-brain barrier once ingested, temporarily altering the chemical milieu of the brain.
Addiction can also be viewed as a continued involvement with a substance or activity despite the negative consequences associated with it. Pleasure and enjoyment would have originally been sought; however, over a period of time involvement with the substance or activity is needed to feel normal. Some psychology professionals and many laypeople now mean 'addiction' to include abnormal psychological dependency on such things as gambling, food, sex, pornography, computers, internet, work, exercise, idolizing, watching TV or certain types of non-pornographic videos, spiritual obsession, self-injury and shopping.
The American Society of Addiction Medicine begins their definition of addiction by describing it as "a primary, chronic disease of brain reward, motivation, memory and related circuitry."


Drug addiction


Drug addiction can simply be defined as a "chronic relapsing disorder characterized by persistent drug-seeking and drug-taking behaviours".






Substance dependence


The American Psychiatric Association's current Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) defines substance dependence as:
"When an individual persists in use of alcohol or other drugs despite problems related to use of the substance, substance dependence may be diagnosed. Compulsive and repetitive use may result in tolerance to the effect of the drug and withdrawal symptoms when use is reduced or stopped. This, along with Substance Abuse are considered Substance Use Disorders...." 
Substance dependence can be diagnosed with physiological dependence, evidence of tolerance or withdrawal, or without physiological dependence.
DSM-IV substance dependencies:
303.90 Alcohol dependence
304.00 Opioid dependence
304.10 Sedative, hypnotic, or anxiolytic dependence (including benzodiazepine dependence and barbiturate dependence)
304.20 Cocaine dependence
304.30 Cannabis dependence
304.40 Amphetamine dependence (or amphetamine-like)
304.50 Hallucinogen dependence
304.60 Inhalant dependence
304.80 Polysubstance dependence
304.90 Phencyclidine (or phencyclidine-like) dependence
304.90 Other (or unknown) substance dependence
305.10 Nicotine dependence




Behavioral addiction


The term addiction is also sometimes applied to compulsions that are not substance-related, such as compulsive shopping, sex addiction/compulsive sex, overeating, problem gambling, exercise/sport and computer addiction. In these kinds of common usages, the term addiction is used to describe a recurring compulsion by an individual to engage in some specific activity, despite harmful consequences, as deemed by the user themselves to their individual health, mental state, or social life. There may be biological and psychological factors contributing to these addictions.