Physical dependence can manifest itself in the appearance of both physical and psychological symptoms which are caused by physiological adaptions in the central nervous system and the brain due to chronic exposure to a substance. Symptoms which may be experienced during withdrawal or reduction in dosage include increased heart rate and/or blood pressure, sweating, and tremors. More serious withdrawal symptoms such as confusion, seizures, and visual hallucinations indicate a serious emergency and the need for immediate medical care. Sedative hypnotic drugs such as alcohol, benzodiazepines, and barbiturates are the only commonly available substances that can be fatal in withdrawal due to their propensity to induce withdrawal convulsions. Abrupt withdrawal from other drugs, such as opioids can cause an extremely physiologically and psychologically painful withdrawal that is very rarely fatal in patients of general good health and with medical treatment, but is more often fatal in patients with weakened cardiovascular systems; toxicity is generally caused by the often-extreme increases in heart rate and blood pressure (which can be treated with clonidine), or due to arrhythmia due to electrolyte imbalance caused by the inability to eat, and constant diarrhea and vomiting (which can be treated with loperamide and ondansetron respectively) associated with acute opioid withdrawal, especially in longer-acting substances where the diarrhea and emesis can continue unabated for weeks, although life-threatening complications are extremely rare, and nearly non-existent with proper medical management.

Cannabis withdrawal symptoms can occur in one half of patients in treatment for cannabis use disorders. These symptoms include dysphoria (anxiety, irritability, depression, restlessness), disturbed sleep, gastrointestinal symptoms, and decreased appetite. Most symptoms begin during the first week of abstinence and resolve after a few weeks.

According to the National Cannabis Prevention and Information Centre in Australia, a sign of cannabis dependence is that an individual spends noticeably more time than the average recreational user recovering from the use of or obtaining cannabis. For some, using cannabis becomes a substantial and disruptive part of an individual's life and he or she may exhibit difficulties in meeting personal obligations or participating in important life activities, preferring to use cannabis instead. People who are cannabis dependent have the inability to stop or decrease using cannabis on their own.

Cannabis use is associated with comorbid mental health problems, such as mood and anxiety disorders, and discontinuing cannabis use is difficult for some users. Psychiatric comorbidities are often present in dependent cannabis users including a range of personality disorders.

Physical dependence is a physical condition caused by chronic use of a tolerance forming drug, in which abrupt or gradual drug withdrawal causes unpleasant physical symptoms. Physical dependence can develop from low-dose therapeutic use of certain medications such as benzodiazepines, opioids, antiepileptics and antidepressants, as well as the recreational misuse of drugs such as alcohol, opioids, and benzodiazepines. The higher the dose used, the greater the duration of use, and the earlier age use began are predictive of worsened physical dependence and thus more severe withdrawal syndromes. Acute withdrawal syndromes can last days, weeks or months. Protracted withdrawal syndrome, also known as post-acute-withdrawal syndrome or "PAWS", is a low-grade continuation of some of the symptoms of acute withdrawal, typically in a remitting-relapsing pattern, often resulting in relapse and prolonged disability of a degree to preclude the possibility of lawful employment. Protracted withdrawal syndrome can last for months, years, or depending on individual factors, indefinitely. Protracted withdrawal syndrome is noted to be most often caused by benzodiazepines. To dispel the popular misassociation with addiction, physical dependence to medications is sometimes compared to dependence on insulin by persons with diabetes.

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.

Research data indicates that steroids affect the serotonin and dopamine neurotransmitter systems of the brain. In an animal study, male rats developed a conditioned place preference to testosterone injections into the nucleus accumbens, an effect blocked by dopamine antagonists, which suggests that androgen reinforcement is mediated by the brain. Moreover, testosterone appears to act through the mesolimbic dopamine system, a common substrate for drugs of abuse. Nonetheless, androgen reinforcement is not comparable to that of cocaine, nicotine, or heroin. Instead, testosterone resembles other mild reinforcers, such as caffeine, or benzodiazepines. The potential for androgen addiction remains to be determined.

Anabolic steroids are not psychoactive and cannot be detected by stimuli devices like a pupilometer which makes them hard to spot as a source of neuropsychological imbalaces in some AAS users.

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."

The Diagnostic Statistical Manual IV (DSM IV) and the International Classification of Diseases, Volume 10 (ICD 10) differ in the way they regard Anabolic-Androgenic Steroids' (AAS) potential for producing dependence.

DSM IV regards AAS as potentially dependence producing. ICD 10 however regards them as non-dependence producing. Anabolic steroids are not physically addictive but users can develop a psychological dependence on the physical result.

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.

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).

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.

Other conditions may commonly co-occur with FAS, stemming from prenatal alcohol exposure. However, these conditions are considered alcohol-related birth defects and not diagnostic criteria for FAS.

- Heart: A heart murmur that frequently disappears by one year of age. Ventricular septal defect most commonly seen, followed by an atrial septal defect.

- Bones: Joint anomalies including abnormal position and function, altered palmar crease patterns, small distal phalanges, and small fifth fingernails.

- Kidneys: Horseshoe, aplastic, dysplastic, or hypoplastic kidneys.

- Eyes: Strabismus, optic nerve hypoplasia (which may cause light sensitivity, decreased visual acuity, or involuntary eye movements).

- Occasional problems: ptosis of the eyelid, microophthalmia, cleft lip with or without a cleft palate, webbed neck, short neck, tetralogy of Fallot, coarctation of the aorta, spina bifida, and hydrocephalus.

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

Cocaine is a powerful stimulant known to make users feel energetic, happy, talkative, etc. In time, negative side effects include increased body temperature, irregular or rapid heart rate, high blood pressure, increased risk of heart attacks, strokes and even sudden death from cardiac arrest. Many habitual abusers develop a transient, manic-like condition similar to amphetamine psychosis and schizophrenia, whose symptoms include aggression, severe paranoia, restlessness, confusion and tactile hallucinations; which can include the feeling of insects under the skin (formication), also known as "coke bugs", during binges. Users of cocaine have also reported having thoughts of suicide, unusual weight loss, trouble maintaining relationships, and an unhealthy, pale appearance.

The key of FASD can vary between individuals exposed to alcohol during pregnancy. While consensus exists for the definition and diagnosis of FAS, minor variations among the systems lead to differences in definitions and diagnostic cut-off criteria for other diagnoses across the FASD continuum. The central nervous system damage criteria particularly lack clear consensus. A working knowledge of the key features is helpful in understanding FASD diagnoses and conditions, and each is reviewed with attention to similarities and differences across the four diagnostic systems. More than 400 problems, however, can occur with FASD.

Excessive computer use may result in, or occur with:

- Lack of face to face social interaction

- Computer vision syndrome

Cocaine dependence is a psychological desire to use cocaine regularly. Cocaine overdose may result in cardiovascular and brain damage, such as: constricting blood vessels in the brain, causing strokes and constricting arteries in the heart; causing heart attacks.

The use of cocaine creates euphoria and high amounts of energy. If taken in large, unsafe doses, it is possible to cause mood swings, paranoia, insomnia, psychosis, high blood pressure, a fast heart rate, panic attacks, cognitive impairments and drastic changes in personality.

The symptoms of cocaine withdrawal (also known as "comedown or crash") range from moderate to severe: dysphoria, depression, anxiety, psychological and physical weakness, pain, and compulsive cravings.

Timmen Cermak, M.D., proposed that co‐dependency be listed as a personality disorder in the Diagnostic and Statistical Manual of Mental Disorders (DSM-III-R; American Psychiatric Association, 1987). Cermak reasoned that when specific personality traits become excessive and maladaptive and caused significant impairment in functioning or caused significant distress, it warrants a personality disorder diagnosis.

Cermak proposed the following criteria for this disorder:

1. Continued investment of self-esteem in the ability to control both oneself and others in the face of serious adverse consequences.

2. Assumption of responsibility for meeting others' needs to the exclusion of acknowledging one's own.

3. Anxiety and boundary distortions around intimacy and separation.

4. Enmeshment in relationships with personality disordered, chemically dependent, other co ‐ dependent, and/or impulse ‐ disordered individuals.

5. Three or more of the following:

1. Excessive reliance on denial

2. Constriction of emotions (with or without dramatic outbursts)

3. Depression

4. Hypervigilance

5. Compulsions

6. Anxiety

7. Substance abuse

8. Has been (or is) the victim of recurrent physical or sexual abuse

9. Stress ‐ related medical illnesses

10. Has remained in a primary relationship with an active substance abuser for at least two years without seeking outside help.

Overmedication is an inappropriate medical treatment that occurs when a patient takes unnecessary or excessive medications. This may happen because the prescriber is unaware of other medications the patient is already taking, because of drug interactions with another chemical or target population, because of human error, because of undiagnosed medical conditions or because of conflicts of interest in the pharmaceutical industry, creating-over promotion (via advertising campaigns, sales to private practice Doctors, or biased or altered medical studies) causing widespread unnecessary use of a specific medicine, or unnecessary dosage of a medicine, due to excessive profit motives in the pharmaceutical industry. This is also sometimes described as the "commercialization of medicine".

Overmedication can also occur when consumers take more medication than is prescribed or as labeled on over-the-counter (OTC) products—either intentionally or unintentionally—or when consumers unknowingly take both prescription and nonprescription drug products containing the same active ingredients. For example, overmedication (in the form of acute overdose) can occur when a prescription drug like Vicodin, which contains both hydrocodone and acetaminophen, is taken along with the nonprescription product Tylenol, which contains acetaminophen as the active ingredient. In other words, overmedication can be caused by both prescribers and consumers or their caretakers.

Another important instance of overmedication occurs when consumers are either prescribed or take additional prescribed or OTC drugs which produce the same or similar therapeutic effects. For instance, if a patient is taking a prescription strength ibuprofen product and also uses a naprosyn product—whether prescription or OTC strength—this, too, can constitute overmedication, can be dangerous, and can be costly to the patient in overall health care costs. Often consumers/patients overmedicate themselves by taking their medications at shorter intervals than prescribed or than container labels specify. As a result, medications may accumulate at higher levels, causing undesired side effects, sometimes serious, or even fatal. Such situations are often reversed through targeted deprescribing by members of the medical team.

Persons who feel that they are overmedicated tend to not to follow their physician's instructions for taking their medication.

Computer addiction can be described as the excessive or compulsive use of the computer which persists despite serious negative consequences for personal, social, or occupational function. Another clear conceptualization is made by Block, who stated that "Conceptually, the diagnosis is a compulsive-impulsive spectrum disorder that involves online and/or offline computer usage and consists of at least three subtypes: excessive gaming, sexual preoccupations, and e-mail/text messaging". While it was expected that this new type of addiction would find a place under the compulsive disorders in the DSM-5, the current edition of the "Diagnostic and Statistical Manual of Mental Disorders", it is still counted as an unofficial disorder. The concept of computer addiction is broadly divided into two types, namely offline computer addiction and online computer addiction. The term offline computer addiction is normally used when speaking about excessive gaming behavior, which can be practiced both offline and online. Online computer addiction, also known as Internet addiction, gets more attention in general from scientific research than offline computer addiction, mainly because most cases of computer addiction are related to the excessive use of the Internet.

Although addiction is usually used to describe dependence on substances, addiction can also be used to describe pathological Internet use. Experts on Internet addiction have described this syndrome as an individual being intensely working on the Internet, prolonged use of the Internet, uncontrollable use of the Internet, unable to use the Internet with efficient time, not being interested in the outside world, not spending time with people from the outside world, and an increase in their loneliness and dejection. However, simply working long hours on the computer does not necessarily mean someone is addicted.

Alcoholism, also known as alcohol use disorder (AUD), is a broad term for any drinking of alcohol that results in mental or physical health problems. The disorder was previously divided into two types: alcohol abuse and alcohol dependence. In a medical context, alcoholism is said to exist when two or more of the following conditions is present: a person drinks large amounts over a long time period, has difficulty cutting down, acquiring and drinking alcohol takes up a great deal of time, alcohol is strongly desired, usage results in not fulfilling responsibilities, usage results in social problems, usage results in health problems, usage results in risky situations, withdrawal occurs when stopping, and alcohol tolerance has occurred with use. Risky situations include drinking and driving or having unsafe sex, among other things. Alcohol use can affect all parts of the body, but it particularly affects the brain, heart, liver, pancreas, and immune system. This can result in mental illness, Wernicke–Korsakoff syndrome, an irregular heartbeat, cirrhosis of the liver, and an increase in the risk of cancer, among other diseases. Drinking during pregnancy can cause damage to the baby resulting in fetal alcohol spectrum disorders. Women are generally more sensitive then men to the harmful physical and mental effects of alcohol.

Environmental factors and genetics are two components that are associated with alcoholism, with about half the risk attributed to each . A person with a parent or sibling with alcoholism is three to four times more likely to become an alcoholic themselves. Environmental factors include social, cultural, and behavioral influences. High stress levels, anxiety, as well as inexpensive cost and easy accessibility to alcohol increase the risk. People may continue to drink partly to prevent or improve symptoms of withdrawal. After a person stops drinking alcohol, they may experience a low level of withdrawal lasting for months. Medically, alcoholism is considered both a physical and mental illness. Questionnaires and certain blood tests may both detect people with possible alcoholism. Further information is then collected to confirm the diagnosis.

Prevention of alcoholism may be attempted by regulating and limiting the sale of alcohol, taxing alcohol to increase its cost, and providing inexpensive treatment. Treatment may take several steps. Due to medical problems that can occur during withdrawal, alcohol detoxification should be carefully controlled. One common method involves the use of benzodiazepine medications, such as diazepam. This can be either given while admitted to a health care institution or occasionally while a person remains in the community with close supervision. Mental illness or other addictions may complicate treatment. After detoxification support such as group therapy or support groups are used to help keep a person from returning to drinking. One commonly used form of support is the group Alcoholics Anonymous. The medications acamprosate, disulfiram, or naltrexone may also be used to help prevent further drinking.

The World Health Organization estimates that as of 2010 there were 208 million people with alcoholism worldwide (4.1% of the population over 15 years of age). In the United States about 17 million (7%) of adults and 0.7 million (2.8%) of those age 12 to 17 years of age are affected. It is more common among males and young adults, becoming less common in middle and old age. It is the least common in Africa at 1.1% and has the highest rates in Eastern Europe at 11%. Alcoholism directly resulted in 139,000 deaths in 2013, up from 112,000 deaths in 1990. A total of 3.3 million deaths (5.9% of all deaths) are believed to be due to alcohol. It often reduces a person's life expectancy by around ten years. In the United States it resulted in economic costs of $224 billion USD in 2006. Many terms, some insulting and others informal, have been used to refer to people affected by alcoholism; the expressions include tippler, drunkard, dipsomaniac, and souse. In 1979, the World Health Organization discouraged the use of "alcoholism" due to its inexact meaning, preferring "alcohol dependence syndrome".

Environmental dependency syndrome, also called Zelig syndrome or Zelig-like syndrome from the name of the protagonist of Woody Allen's "Zelig", is a syndrome where the affected individual relies on environmental cues in order to accomplish goals or tasks. It is a disorder in personal autonomy that is influenced by individual psychological traits and can be helped through the intervention of other people. For example, adults diagnosed with attention deficit hyperactivity disorder have relied on special coaches to provide cues at appropriate times, helping them to make decisions about how to prioritize and order tasks.

Alcohol tolerance refers to the bodily responses to the functional effects of ethanol in alcoholic beverages. This includes direct tolerance, speed of recovery from insobriety and resistance to the development of alcoholism.

Warning signs of alcoholism include the consumption of increasing amounts of alcohol and frequent intoxication, preoccupation with drinking to the exclusion of other activities, promises to quit drinking and failure to keep those promises, the inability to remember what was said or done while drinking (colloquially known as "blackouts"), personality changes associated with drinking, denial or the making of excuses for drinking, the refusal to admit excessive drinking, dysfunction or other problems at work or school, the loss of interest in personal appearance or hygiene, marital and economic problems, and the complaint of poor health, with loss of appetite, respiratory infections, or increased anxiety.