Substance dependence, also known as drug dependence, is a biopsychological situation whereby an individual's functionality is dependent on the necessitated re-consumption of a psychoactive substance because of an adaptive state that has developed within the individual from psychoactive substance consumption that results in the experience of withdrawal and that necessitates the re-consumption of the drug.[1][2] A drug addiction, a distinct concept from substance dependence, is defined as compulsive, out-of-control drug use, despite negative consequences.[1][2] An addictive drug is a drug which is both rewarding and reinforcing.[1] ΔFosB, a gene transcription factor, is now known to be a critical component and common factor in the development of virtually all forms of behavioral and drug addictions,[3][4][5] but not dependence.
The International Classification of Diseases classifies substance dependence as a mental and behavioural disorder.[6]Within the framework of the 4th edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV), substance dependence is redefined as a drug addiction, and can be diagnosed without the occurrence of a withdrawal syndrome.[7] It was described accordingly: "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."[8] In the DSM-5 (released in 2013), substance abuse and substance dependence have been merged into the category of substance use disorders and they no longer exist as individual diagnoses.[9]
Withdrawal is the body's reaction to abstaining from a substance upon which a person has developed a dependence syndrome. When dependence has developed, cessation of substance-use produces an unpleasant state, which promotes continued drug use through negative reinforcement; i.e., the drug is used to escape or avoid re-entering the associated withdrawal state. The withdrawal state may include physical-somatic symptoms (physical dependence), emotional-motivational symptoms (psychological dependence), or both. Chemical and hormonal imbalances may arise if the substance is not re-introduced. Psychological stress may also result if the substance is not re-introduced.[13]
Infants also experience substance withdrawal, known as neonatal abstinence syndrome (NAS), which can have severe and life-threatening effects. Addiction to drugs such as alcohol in expectant mothers not only causes NAS, but also an array of other issues which can continually affect the infant throughout their lifetime.[14]
The dependence potential or dependence liability of a drug varies from substance to substance, and from individual to individual. Dose, frequency, pharmacokinetics of a particular substance, route of administration, and time are critical factors for developing a drug dependence.
An article in The Lancet compared the harm and dependence liability of 20 drugs, using a scale from zero to three for physical dependence, psychological dependence, and pleasure to create a mean score for dependence. Selected results can be seen in the chart below.[15]