Addiction is a biopsychosocial disorder characterized by compulsive engagement in rewarding stimuli despite adverse consequences.[3][5][2][6][7][8] Despite the involvement of a number of psychosocial factors, a biological process—one that is induced by repeated exposure to an addictive stimulus—is the core pathology that drives the development and maintenance of an addiction, according to the "brain disease model" of addiction.[3] However, some scholars who study addiction argue that the brain disease model is incomplete and misleading.[9][10][11][12][13][14]

Other namesSevere substance use disorder[1][2]
Brain positron emission tomography images that compare brain metabolism in a healthy individual and an individual with a cocaine addiction
Addiction and dependence glossary[3][4][5][2]
  • addiction – a biopsychosocial disorder characterized by persistent use of drugs (including alcohol) despite substantial harm and adverse consequences
  • addictive drug – psychoactive substances that with repeated use are associated with significantly higher rates of substance use disorders, due in large part to the drug's effect on brain reward systems
  • dependence – an adaptive state associated with a withdrawal syndrome upon cessation of repeated exposure to a stimulus (e.g., drug intake)
  • drug sensitization or reverse tolerance – the escalating effect of a drug resulting from repeated administration at a given dose
  • drug withdrawal – symptoms that occur upon cessation of repeated drug use
  • physical dependence – dependence that involves persistent physical–somatic withdrawal symptoms (e.g., fatigue and delirium tremens)
  • psychological dependence – dependence that involves emotional–motivational withdrawal symptoms (e.g., dysphoria and anhedonia)
  • reinforcing stimuli – stimuli that increase the probability of repeating behaviors paired with them
  • rewarding stimuli – stimuli that the brain interprets as intrinsically positive and desirable or as something to approach
  • sensitization – an amplified response to a stimulus resulting from repeated exposure to it
  • substance use disorder – a condition in which the use of substances leads to clinically and functionally significant impairment or distress
  • tolerance – the diminishing effect of a drug resulting from repeated administration at a given dose

The brain disease model posits that addiction is a disorder of the brain's reward system which arises through transcriptional and epigenetic mechanisms and develops over time from chronically high levels of exposure to an addictive stimulus (e.g., eating food, the use of cocaine, engagement in sexual activity, participation in high-thrill cultural activities such as gambling, etc.).[3][15][16] DeltaFosB (ΔFosB), a gene transcription factor, is a critical component and common factor in the development of virtually all forms of behavioral and drug addictions.[15][16][17][18] Two decades of research into ΔFosB's role in addiction have demonstrated that addiction arises, and the associated compulsive behavior intensifies or attenuates, along with the overexpression of ΔFosB in the D1-type medium spiny neurons of the nucleus accumbens.[3][15][16][17] Due to the causal relationship between ΔFosB expression and addictions, it is used preclinically as an addiction biomarker.[3][15][17] ΔFosB expression in these neurons directly and positively regulates drug self-administration and reward sensitization through positive reinforcement, while decreasing sensitivity to aversion.[note 1][3][15]

Addiction exacts an "astoundingly high financial and human toll" on individuals and society as a whole.[19][20][21] In the United States, the total economic cost to society is greater than that of all types of diabetes and all cancers combined.[21] These costs arise from the direct adverse effects of drugs and associated healthcare costs (e.g., emergency medical services and outpatient and inpatient care), long-term complications (e.g., lung cancer from smoking tobacco products, liver cirrhosis and dementia from chronic alcohol consumption, and meth mouth from methamphetamine use), the loss of productivity and associated welfare costs, fatal and non-fatal accidents (e.g., traffic collisions), suicides, homicides, and incarceration, among others.[19][20][21][22] Classic hallmarks of addiction include impaired control over substances or behavior, preoccupation with substance or behavior, and continued use despite consequences.[23] Habits and patterns associated with addiction are typically characterized by immediate gratification (short-term reward), coupled with delayed deleterious effects (long-term costs).[24]

The etymology of addiction throughout history has been often misunderstood and has taken on various meanings associated with the word. An example is the usage of the word during the Early Modern period. ‘Addiction’ at the time, meant to ‘attach’ to something, giving it both positive and negative connotations. The object of this attachment could be characterised as “good or bad”.,[25] however, the meaning of addiction during this period was mostly associated with positivity and goodness. During the highly religious era, it was seen as a way of “devoting oneself to another”.[25] Modern research on addiction has led to a better understanding of the disease with research studies on the topic dating back to 1875, specifically on morphine addiction.[26] This furthered the understanding of addiction being a medical condition. It wasn’t until the 19th century that addiction was seen and acknowledged as a disease, being both a medical and mental illness.[27]
Today, addiction is understood as a disease that negatively impacts those who are diagnosed, most commonly associated with drug and alcohol abuse. The understanding of addiction has changed through-out history, which has impacted, and continues to impact the ways it is medically treated and diagnosed.

Examples of drug and behavioral addictions include alcoholism, marijuana addiction, amphetamine addiction, cocaine addiction, nicotine addiction, opioid addiction, food addiction, chocolate addiction, video game addiction, gambling addiction, and sexual addiction. The only behavioral addiction recognized by the DSM-5 and the ICD-10 is gambling addiction. With the introduction of the ICD-11 gaming addiction was appended.[28] The term "addiction" is frequently misused when referring to other compulsive behaviors or disorders, particularly dependence, in news media.[29] An important distinction between drug addiction and dependence is that drug dependence is a disorder in which cessation of drug use results in an unpleasant state of withdrawal, which can lead to further drug use.[30] Addiction is the compulsive use of a substance or performance of a behavior that is independent of withdrawal. Addiction can occur in the absence of dependence, and dependence can occur in the absence of addiction, although the two often occur together.