Common symptoms include:

- Sudden changes in behaviour – may engage in secretive or suspicious behaviour

- Mood changes – anger towards others, paranoia and little care shown about themselves or their future

- Problems with work or school – lack of attendance

- Changes in eating and sleeping habits

- Changes in friendship groups and poor family relationships

- A sudden unexplained change in financial needs – leading to borrowing/stealing money

There are many more symptoms such as physical and psychological changes, though this is often dependent on which drug is being abused. It is, however, common that abusers will experience unpleasant withdrawal symptoms if the drug is taken away from them.

It is also reported that others have strong cravings even after they have not used the drug for a long period of time. This is called being "clean". To determine how the brain triggers these cravings, multiple tests have been done on mice. It is also now thought that these cravings can be explained by substance-related disorders as a subcategory of personality disorders as classified by the DSM-5.

Substance abuse, also known as drug abuse, is a patterned use of a substance (drug) in which the user consumes the substance in amounts or with methods which are harmful to themselves or others.

The drugs used are often associated with levels of intoxication that alter judgment, perception, attention and physical control, not related with medical or therapeutic effects. It is often thought that the main abused substances are illegal drugs and alcohol; however it is becoming more common that prescription drugs and tobacco are a prevalent problem.

Substance-related disorders, including both substance dependence and substance abuse, can lead to large societal problems. It is found to be greatest in individuals ages 18–25, with a higher likelihood occurring in men compared to women, and urban residents compared to rural residents. On average, general medical facilities hold 20% of patients with substance-related disorders, possibly leading to psychiatric disorders later on. Over 50% of individuals with substance-related disorders will often have a "dual diagnosis," where they are diagnosed with the substance abuse, as well as a psychiatric diagnosis, the most common being major depression, personality disorder, anxiety disorders, and dysthymia.

The DSM definition of addiction can be boiled down to compulsive use of a substance (or engagement in an activity) despite ongoing negative consequences. The medical community makes a distinction between physical dependence (characterized by symptoms of physical withdrawal symptoms, like tremors and sweating) and psychological dependence (emotional-motivational withdrawal symptoms). Physical dependence is simply needing a substance to function. Humans are all physically dependent upon oxygen, food and water. A drug can cause physical dependence and not psychological dependence (for example, some blood pressure medications, which can produce fatal withdrawal symptoms if not tapered) and some can cause psychological dependence without physical dependence (the withdrawal symptoms associated with cocaine are all psychological, there is no associated vomiting or diarrhea as there is with opiate withdrawal).

There are several different screening tools that have been validated for use with adolescents such as the CRAFFT and adults such as the CAGE.

Physical dependence on a substance is defined by the appearance of characteristic physical withdrawal symptoms when the substance is suddenly discontinued. Opiates, benzodiazepines, barbiturates, alcohol and nicotine induce physical dependence. On the other hand, some categories of substances share this property and are still not considered addictive: cortisone, beta blockers and most antidepressants are examples.

Some substances induce physical dependence or physiological tolerance - but not addiction — for example many laxatives, which are not psychoactive; nasal decongestants, which can cause rebound congestion if used for more than a few days in a row; and some antidepressants, most notably venlafaxine, paroxetine and sertraline, as they have quite short half-lives, so stopping them abruptly causes a more rapid change in the neurotransmitter balance in the brain than many other antidepressants. Many non-addictive prescription drugs should not be suddenly stopped, so a doctor should be consulted before abruptly discontinuing them.

The speed with which a given individual becomes addicted to various substances varies with the substance, the frequency of use, the means of ingestion, the intensity of pleasure or euphoria, and the individual's genetic and psychological susceptibility. Some people may exhibit alcoholic tendencies from the moment of first intoxication, while most people can drink socially without ever becoming addicted. Opioid dependent individuals have different responses to even low doses of opioids than the majority of people, although this may be due to a variety of other factors, as opioid use heavily stimulates pleasure-inducing neurotransmitters in the brain. Nonetheless, because of these variations, in addition to the adoption and twin studies that have been well replicated, much of the medical community is satisfied that addiction is in part genetically moderated. That is, one's genetic makeup may regulate how susceptible one is to a substance and how easily one may become attached to a pleasurable routine.

Eating disorders are complicated pathological mental illnesses and thus are not the same as addictions described in this article. Eating disorders, which some argue are not addictions at all, are driven by a multitude of factors, most of which are highly different from the factors behind addictions described in this article. It has been reported, however, that patients with eating disorders can successfully be treated with the same non-pharmacological protocols used in patients with chemical addiction disorders.

Gambling is another potentially addictive behavior with some biological overlap. Conversely gambling urges have emerged with the administration of Mirapex (pramipexole), a dopamine agonist.

The obsolete term physical addiction is deprecated, because of its connotations. In modern pain management with opioids physical dependence is nearly universal. High-quality, long-term studies are needed to better delineate the risks and benefits of chronic opiate use.

Signs and symptoms include:

- Drug seeking behavior

- Multiple prescriptions from different providers

- Increased use over time

- Opioid cravings

- Multiple medical complications from drug use (HIV/AIDS, hospitalizations, abscesses)

- Legal or social ramifications secondary to drug use

- Withdrawal symptoms

Signs and symptoms of opioid intoxication include:

- Decreased perception of pain

- Euphoria

- Confusion

- Desire to sleep

- Nausea

- Constipation

- Miosis

Behavioral addiction is a form of addiction that involves a compulsion to engage in a rewarding non-drug-related behavior – sometimes called a natural reward – despite any negative consequences to the person's physical, mental, social or financial well-being. A gene transcription factor known as ΔFosB has been identified as a necessary common factor involved in both behavioral and drug addictions, which are associated with the same set of neural adaptations in the reward system.

Addiction is defined by Webster Dictionary as a "compulsive need for and use of a habit-forming substance characterized by tolerance and by well-defined physiological symptoms upon withdrawal; broadly: persistent compulsive use of a substance known by the user to be harmful".

Problematic Internet use is also called compulsive Internet use (CIU), Internet overuse, problematic computer use, or pathological computer use (PCU), problematic Internet use (PIU), or Internet addiction disorder (IAD)). Another commonly associated pathology is video game addiction, or Internet gaming disorder (IGD).

Dual diagnosis (also called co-occurring disorders, COD, or dual pathology) is the condition of suffering from a mental illness and a comorbid substance abuse problem. There is considerable debate surrounding the appropriateness of using a single category for a heterogeneous group of individuals with complex needs and a varied range of problems. The concept can be used broadly, for example depression and alcoholism, or it can be restricted to specify severe mental illness (e.g. psychosis, schizophrenia) and substance misuse disorder (e.g. cannabis abuse), or a person who has a milder mental illness and a drug dependency, such as panic disorder or generalized anxiety disorder and is dependent on opioids. Diagnosing a primary psychiatric illness in substance abusers is challenging as drug abuse itself often induces psychiatric symptoms, thus making it necessary to differentiate between substance induced and pre-existing mental illness.

Those with co-occurring disorders face complex challenges. They have increased rates of relapse, hospitalization, homelessness, and HIV and hepatitis C infection compared to those with either mental or substance use disorders alone. The cause of co-occurring disorders is unknown, although there are several theories.

From the ICD-9 database:

- A chronic disease in which a person craves drinks that contain alcohol and is unable to control his or her drinking. A person with this disease also needs to drink greater amounts to get the same effect and has withdrawal symptoms after stopping alcohol use. Alcoholism affects physical and mental health, and can cause problems with family, friends, and work.

- A disorder characterized by a pathological pattern of alcohol use that causes a serious impairment in social or occupational functioning.

- A primary, chronic disease with genetic, psychosocial, and environmental factors influencing its development and manifestations. The disease is often progressive and fatal. It is characterized by impaired control over drinking, preoccupation with the drug alcohol, use of alcohol despite adverse consequences, and distortions in thinking, most notably denial. Each of these symptoms may be continuous or periodic. (morse & flavin for the joint commission of the national council on alcoholism and drug dependence and the american society of addiction medicine to study the definition and criteria for the diagnosis of alcoholism: in jama 1992;268:1012-4)

- For most adults, moderate alcohol use is probably not harmful. However, about 18 million adult Americans are alcoholics or have alcohol problems. Alcoholism is a disease with four main features:

- craving - a strong need to drink

- loss of control - not being able to stop drinking once you've started

- physical dependence - withdrawal symptoms, such as nausea, sweating, or shakiness when you don't drink

- tolerance - the need to drink greater amounts of alcohol to feel the same effect

- Temporary mental disturbance marked by muscle incoordination and paresis as the result of excessive alcohol ingestion.

Internet addiction disorder, more commonly called problematic Internet use (PIU), refers to excessive Internet use that interferes with daily life.

Substance dependence also known as drug dependence is an adaptive state that develops from repeated drug administration, and which results in withdrawal upon cessation of drug use. A "drug addiction", a distinct concept from substance dependence, is defined as compulsive, out-of-control drug use, despite negative consequences. An "addictive drug" is a drug which is both rewarding and reinforcing. 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 addiction and drug addictions, but not dependence.

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. It is now 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."

Five indicators of exercise addiction are:

1. An increase in exercise that may be labeled as detrimental, or becomes harmful.

2. A desire to experience euphoria; exercise may be increased as tolerance of the euphoric state increases.

3. Not participating in physical activity will cause in one's daily life.

4. Severe withdrawal symptoms following exercise deprivation including anxiety, restlessness, depression, guilt, tension, discomfort, loss of appetite, sleeplessness, and headaches.

5. Exercising through trauma and despite physical injuries.

Key differences between healthy and addictive levels of exercise include the presence of withdrawal symptoms when exercise is stopped as well as the addictive properties exercise may have leading to a dependence on exercise.

Impulsivity is characterized by actions based on sudden desires, whims, or inclinations rather than careful thought. Individuals with substance abuse have higher levels of impulsivity, and individuals who use multiple drugs tend to be more impulsive. A number of studies using the Iowa gambling task as a measure for impulsive behavior found that drug using populations made more risky choices compared to healthy controls. There is a hypothesis that the loss of impulse control may be due to impaired inhibitory control resulting from drug induced changes that take place in the frontal cortex. The neurodevelopmental and hormonal changes that happen during adolescence may modulate impulse control that could possibly lead to the experimentation with drugs and may lead to the road of addiction. Impulsivity is thought to be a facet trait in the neuroticism personality domain (overindulgence/negative urgency) which is prospectively associated with the development of substance abuse.

Experts describe the spectrum of behaviors designated as addictive in terms of five interrelated concepts: patterns, habits, compulsions, impulse control disorders, and physical addiction.

A concrete classification of exercise addiction has proven to be difficult due to the lack of a specific and widely accepted diagnostic model. Most interpretations of addiction have traditionally been limited to drugs and alcohol, which makes it even more difficult to identify addictive tendencies in exercise. While excessive exercise is the overarching theme with exercise addiction, the term also includes a variety of symptoms like withdrawal, "exercise buzz", and impaired physical function. Excessive exercise has been classified in different ways; sometimes as an addiction and sometimes as a more general compulsive behavior. Psychiatric case studies have shown that exaggerated exercise could lead to negligence of work and family life. With an addiction, individuals become "hooked" to the feeling of euphoria and pleasure that exercise provides. This pleasure keeps the individual from stopping and leads to excessive exercise. With a compulsion people often do not necessarily enjoy repeating certain tasks, as they may feel like performing it will fulfill a duty that is required of them. There are many opinions on whether concrete diagnostic criteria should be created for this type of addiction. Some say preoccupation with exercise that causes significant impairment in a person's life, not due to another disorder, may be enough criteria to label this disorder. Others say there is not enough information about exercise addiction to develop diagnostic criteria. , the term "excessive exercise" continues to be used while the "exercise addiction" model continues to be debated.

Three main types of disorders are associated with excessive exercise:

1. Anorexia athletica (obligatory exercise) - When an individual feels compelled to exercise beyond the point of benefitting one's body. Individuals will participate in athletic activities regardless of pain, injury, illness, etc., and will try to arrange their lives in order to maximize workout time.

2. Exercise bulimia - When an individual has binge eating sessions that are followed by periods of high-intensity exercise.

3. Body dysmorphic disorder - When an individual is obsessed with parts of their body and perceive them to be different or odd. These individuals will create highly regimented routines in order to improve their perception of the "flawed" body part.

Depending on the actual compound, drug abuse including alcohol may lead to health problems, social problems, morbidity, injuries, unprotected sex, violence, deaths, motor vehicle accidents, homicides, suicides, physical dependence or psychological addiction.

There is a high rate of suicide in alcoholics and other drug abusers. The reasons believed to cause the increased risk of suicide include the long-term abuse of alcohol and other drugs causing physiological distortion of brain chemistry as well as the social isolation. Another factor is the acute intoxicating effects of the drugs may make suicide more likely to occur. Suicide is also very common in adolescent alcohol abusers, with 1 in 4 suicides in adolescents being related to alcohol abuse. In the USA approximately 30% of suicides are related to alcohol abuse. Alcohol abuse is also associated with increased risks of committing criminal offences including child abuse, domestic violence, rapes, burglaries and assaults.

Drug abuse, including alcohol and prescription drugs, can induce symptomatology which resembles mental illness. This can occur both in the intoxicated state and also during the withdrawal state. In some cases these substance induced psychiatric disorders can persist long after detoxification, such as prolonged psychosis or depression after amphetamine or cocaine abuse. A protracted withdrawal syndrome can also occur with symptoms persisting for months after cessation of use. Benzodiazepines are the most notable drug for inducing prolonged withdrawal effects with symptoms sometimes persisting for years after cessation of use. Abuse of hallucinogens can trigger delusional and other psychotic phenomena long after cessation of use.

Cannabis may trigger panic attacks during intoxication and with continued use, it may cause a state similar to dysthymia. Researchers have found that daily cannabis use and the use of high-potency cannabis are independently associated with a higher chance of developing schizophrenia and other psychotic disorders.

Severe anxiety and depression are commonly induced by sustained alcohol abuse, which in most cases abates with prolonged abstinence. Even sustained moderate alcohol use may increase anxiety and depression levels in some individuals. In most cases these drug induced psychiatric disorders fade away with prolonged abstinence.

According to the DSM-IV criteria for alcohol dependence, at least three out of seven of the following criteria must be manifest during a 12-month period:

- Tolerance

- Withdrawal symptoms or clinically defined alcohol withdrawal syndrome

- Use in larger amounts or for longer periods than intended

- Persistent desire or unsuccessful efforts to cut down on alcohol use

- Time is spent obtaining alcohol or recovering from effects

- Social, occupational and recreational pursuits are given up or reduced because of alcohol use

- Use is continued despite knowledge of alcohol-related harm (physical or psychological)

The signs and symptoms of benzodiazepine dependence include feeling unable to cope without the drug, unsuccessful

attempts to cut down or stop benzodiazepine use, tolerance to the effects of benzodiazepines, and withdrawal symptoms when not taking the drug. Some withdrawal symptoms that may appear include anxiety, depressed mood, depersonalisation, derealisation, sleep disturbance, hypersensitivity to touch and pain, tremor, shakiness, muscular aches, pains, twitches, and headache. Benzodiazepine dependence and withdrawal have been associated with suicide and self-harming behaviors, especially in young people. The Department of Health substance misuse guidelines recommend monitoring for mood disorder in those dependent on or withdrawing from benzodiazepines.

Benzodiazepine dependence is a frequent complication for those prescribed for or using for longer than four weeks, with physical dependence and withdrawal symptoms being the most common problem, but also occasionally drug-seeking behavior. Withdrawal symptoms include anxiety, perceptual disturbances, distortion of all the senses, dysphoria, and, in rare cases, psychosis and epileptic seizures.

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 introduced. Psychological stress may also result if the substance is not re-introduced.

Infants also suffer from substance withdrawal, known as Neonnatal Abstinence Syndrome (NAS) which has severe and life-threatening effects on growing fetus. Addiction to drugs and alcohol in expecting mothers does not only cause NAS but also an array of other issues which can continually affect the infant throughout his/her lifetime. The type of drug which was abused during the months of pregnancy has many different effects on the child which can affect the infant in many ways throughout his/her life.

Some of the fundamental components of kleptomania include recurring intrusive thoughts, impotence to resist the compulsion to engage in stealing, and the release of pressure following the act. These symptoms suggest that kleptomania could be regarded as an obsessive-compulsive type of disorder.

People diagnosed with kleptomania often have other types of disorders involving mood, anxiety, eating, impulse control, and drug use. They also have great levels of stress, guilt, and remorse, and privacy issues accompanying the act of stealing. These signs are considered to either cause or intensify general comorbid disorders. The characteristics of the behaviors associated with stealing could result in other problems as well, which include social segregation and substance abuse. The many types of other disorders frequently occurring along with kleptomania usually make clinical diagnosis uncertain.

There is a difference between ordinary theft and kleptomania: "ordinary theft (whether planned or impulsive) is deliberate and is motivated by the usefulness of the object or its monetary worth," whereas with kleptomania, there "is the recurrent failure to resist impulses to steal items even though the items are not needed for personal use or for their monetary value."

Compulsions and addictions are intertwined and reward is one major distinction between an addiction and a compulsion (as it is experienced in obsessive-compulsive disorder). An addiction is, by definition, a form of compulsion, and both addictions and compulsions involve operant reinforcement; however, in addiction, the desire and motivation to use a substance or engage in a behavior arises because it is rewarding (i.e., the compulsions that occur in addiction develop through positive reinforcement). In contrast, someone who experiences a compulsion as part of obsessive-compulsive disorder may not perceive anything rewarding from acting on the compulsion. Often, it is a way of dealing with the obsessive part of the disorder, resulting in a feeling of relief (i.e., compulsions may also arise through negative reinforcement).

Deep brain stimulation to the nucleus accumbens, a region in the brain involved heavily in addiction and reinforcement learning, has proven to be an effective treatment of obsessive compulsive disorder.

Kleptomania or klopemania is the inability to refrain from the urge for stealing items and is usually done for reasons other than personal use or financial gain. First described in 1816, kleptomania is classified in psychiatry as an impulse control disorder. Some of the main characteristics of the disorder suggest that kleptomania could be an obsessive-compulsive spectrum disorder.

The disorder is frequently under-diagnosed and is regularly associated with other psychiatric disorders, particularly anxiety and eating disorders, and alcohol and substance abuse. Patients with kleptomania are typically treated with therapies in other areas due to the comorbid grievances rather than issues directly related to kleptomania.

Over the last 100 years, a shift from psychotherapeutic to psychopharmacological interventions for kleptomania has occurred. Pharmacological treatments using selective serotonin reuptake inhibitors (SSRIs), mood stabilizers and opioid receptor antagonists, and other antidepressants along with cognitive behavioral therapy, have yielded positive results.

Not all mental health professionals agree about standard methods of treatment. Caring for an individual with a physical addiction is not necessarily treating a pathology. The caregiver may only require assertiveness skills and the ability to place responsibility for the addiction on the other. There are various recovery paths for individuals who struggle with codependency. For example, some may choose cognitive-behavioral psychotherapy, sometimes accompanied by chemical therapy for accompanying depression. There also exist support groups for codependency, such as Co-Dependents Anonymous (CoDA), Al-Anon/Alateen, Nar-Anon, and Adult Children of Alcoholics (ACoA), which are based on the twelve-step program model of Alcoholics Anonymous and Celebrate Recovery a Christian, Bible-based group. Many self-help guides have been written on the subject of codependency.

Sometimes an individual can, in attempts to recover from codependency, go from being overly passive or overly giving to being overly aggressive or excessively selfish. Many therapists maintain that finding a balance through healthy assertiveness (which leaves room for being a caring person and also engaging in healthy caring behavior) is true recovery from codependency and that becoming extremely selfish, a bully, or an otherwise conflict-addicted person is not. Developing a permanent stance of being a victim (having a victim mentality) would also not constitute true recovery from codependency and could be another example of going from one extreme to another. A victim mentality could also be seen as a part of one's original state of codependency (lack of empowerment causing one to feel like the "subject" of events rather than being an empowered actor). Someone truly recovered from codependency would feel empowered and like an author of their life and actions rather than being at the mercy of outside forces. A victim mentality may also occur in combination with passive–aggressive control issues. From the perspective of moving beyond victim-hood, the capacity to forgive and let go (with exception of cases of very severe abuse) could also be signs of real recovery from codependency, but the willingness to endure further abuse would not.

Unresolved patterns of codependency can lead to more serious problems like alcoholism, drug addiction, eating disorders, sex addiction, psychosomatic illnesses, and other self-destructive or self-defeating behaviors. People with codependency are also more likely to attract further abuse from aggressive individuals, more likely to stay in stressful jobs or relationships, less likely to seek medical attention when needed and are also less likely to get promotions and tend to earn less money than those without codependency patterns. For some people, the social insecurity caused by codependency can progress into full-blown social anxiety disorders like social phobia, avoidant personality disorder or painful shyness. Other stress-related disorders like panic disorder, depression or PTSD may also be present.

Research by governments in Australia led to a universal definition for that country which appears to be the only research-based definition not to use diagnostic criteria: "Problem gambling is characterized by many difficulties in limiting money and/or time spent on gambling which leads to adverse consequences for the gambler, others, or for the community." The University of Maryland Medical Center defines pathological gambling as "being unable to resist impulses to gamble, which can lead to severe personal or social consequences".

Most other definitions of problem gambling can usually be simplified to any gambling that causes harm to the gambler or someone else in any way; however, these definitions are usually coupled with descriptions of the type of harm or the use of diagnostic criteria. The "DSM-V" has since reclassified pathological gambling as "gambling disorder" and has listed the disorder under substance-related and addictive disorders rather than impulse-control disorders. This is due to the symptomatology of the disorder resembling an addiction not dissimilar to that of substance-abuse. There are both environmental and genetic factors that can influence on gambler and cause some type of addiction. In order to be diagnosed, an individual must have at least four of the following symptoms in a 12-month period:

- Needs to gamble with increasing amounts of money in order to achieve the desired excitement

- Is restless or irritable when attempting to cut down or stop gambling

- Has made repeated unsuccessful efforts to control, cut back, or stop gambling

- Is often preoccupied with gambling (e.g., having persistent thoughts of reliving past gambling experiences, handicapping or planning the next venture, thinking of ways to get money with which to gamble)

- Often gambles when feeling distressed (e.g., helpless, guilty, anxious, depressed)

- After losing money gambling, often returns another day to get even ("chasing" one's losses)

- Lies to conceal the extent of involvement with gambling

- Has jeopardized or lost a significant relationship, job, education or career opportunity because of gambling

- Relies on others to provide money to relieve desperate financial situations caused by gambling