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

Include the following:

- Depression

- Shaking

- Feeling unreal

- Appetite loss

- Muscle twitching

- Memory loss

- Motor impairment

- Nausea

- Muscle pains

- Dizziness

- Apparent movement of still objects

- Feeling faint

- Noise sensitivity

- Light sensitivity

- Peculiar taste

- Pins and needles

- Touch sensitivity

- Sore eyes

- Hallucinations

- Smell sensitivity

All sedative-hypnotics, e.g. alcohol, barbiturates, benzodiazepines and the nonbenzodiazepine Z-drugs have a similar mechanism of action, working on the GABA receptor complex and are cross tolerant with each other and also have abuse potential. Use of prescription sedative-hypnotics; for example the nonbenzodiazepine Z-drugs often leads to a relapse back into substance misuse with one author stating this occurs in over a quarter of those who have achieved abstinence.

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 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.

Sedative-hypnotics such as alcohol, benzodiazepines, and the barbiturates are known for the severe physical dependence that they are capable of inducing which can result in severe withdrawal effects. This severe neuroadaptation is even more profound in high dose drug users and misusers. A high degree of tolerance often occurs in chronic benzodiazepine abusers due to the typically high doses they consume which can lead to a severe benzodiazepine dependence. The benzodiazepine withdrawal syndrome seen in chronic high dose benzodiazepine abusers is similar to that seen in therapeutic low dose users but of a more severe nature. Extreme antisocial behaviors in obtaining continued supplies and severe drug-seeking behavior when withdrawals occur. The severity of the benzodiazepine withdrawal syndrome has been described by one benzodiazepine drug misuser who stated that I'd rather withdraw off heroin any day. If I was withdrawing from benzos you could offer me a gram of heroin or just 20mg of diazepam and I'd take the diazepam every time – I've never been so frightened in my life. Those who use benzodiazepines intermittently are less likely to develop a dependence and withdrawal symptoms upon dose reduction or cessation of benzodiazepines than those who use benzodiazepines on a daily basis.

Misuse of benzodiazepines is widespread amongst drug misusers; however, many of these people will not require withdrawal management as their use is often restricted to binges or occasional misuse. Benzodiazepine dependence when it occurs requires withdrawal treatment. There is little evidence of benefit from long-term substitution therapy of benzodiazepines, and conversely, there is growing evidence of the harm of long-term use of benzodiazepines, especially higher doses. Therefore, gradual reduction is recommended, titrated against withdrawal symptoms. For withdrawal purposes, stabilisation with a long-acting agent such as diazepam is recommended before commencing withdrawal. Chlordiazepoxide (Librium), a long-acting benzodiazepine, is gaining attention as an alternative to diazepam in substance abusers dependent on benzodiazepines due to its decreased abuse potential. In individuals dependent on benzodiazepines who have been using benzodiazepines long-term, taper regimens of 6–12 months have been recommended and found to be more successful. More rapid detoxifications e.g. of a month are not recommended as they lead to more severe withdrawal symptoms.

Tolerance leads to a reduction in GABA receptors and function; when benzodiazepines are reduced or stopped this leads to an unmasking of these compensatory changes in the nervous system with the appearance of physical and mental withdrawal effects such as anxiety, insomnia, autonomic hyperactivity and possibly 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.

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.

Amphetamine dependence refers to a state of psychological dependence on a drug in the amphetamine class. In individuals with substance use disorder (problematic use or abuse with dependence), psychotherapy is currently the best treatment option as no pharmacological treatment has been approved. Tolerance is expected to develop with regular substituted amphetamine use. When substituted amphetamines are abused, drug tolerance develops rapidly.

Severe withdrawal associated with dependence from recreational substituted amphetamine use can be difficult for a user to cope with. Long-term use of certain substituted amphetamines, particularly methamphetamine, can reduce dopamine activity in the brain. Psychostimulants that increase dopamine and mimic the effects of substituted amphetamines, but with lower abuse liability, could theoretically be used as replacement therapy in amphetamine dependence. However, the few studies that used amphetamine, bupropion, methylphenidate and modafinil as a replacement therapy did not result in less methamphetamine use or craving.

In 2013, overdose on amphetamine, methamphetamine, and other compounds implicated in an "amphetamine use disorder" resulted in an estimated 3,788 deaths worldwide (3,425–4,145 deaths, 95% confidence).

Barbiturate dependence develops with regular use of barbiturates. This in turn may lead to a need for increasing doses of the drug to get the original desired pharmacological or therapeutic effect. Barbiturate use can lead to both addiction and physical dependence, and as such they have a high potential for abuse. Management of barbiturate dependence involves considering the affected person's age, comorbidity and the pharmacological pathways of barbiturates. Psychological addiction to barbiturates can develop quickly. The GABA receptor, one of barbiturates' main sites of action, is thought to play a pivotal role in the development of tolerance to and dependence on barbiturates, as well as the euphoric "high" that results from their abuse. The mechanism by which barbiturate tolerance develops is believed to be different from that of ethanol or benzodiazepines, even though these drugs have been shown to exhibit cross-tolerance with each other. The management of a physical dependence on barbiturates is stabilisation on the long-acting barbiturate phenobarbital followed by a gradual titration down of dose. The slowly eliminated phenobarbital lessens the severity of the withdrawal syndrome and reduces the chances of serious barbiturate withdrawal effects such as seizures. Antipsychotics are not recommended for barbiturate withdrawal (or other CNS depressant withdrawal states) especially clozapine, olanzapine or low potency phenothiazines e.g. chlorpromazine as they lower the seizure threshold and can worsen withdrawal effects; if used extreme caution is required.

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.

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.

A psychoactive drug, such as a stimulant, is a chemical or substance that effects one’s behavior, mind, and body. A stimulant can be smoked, injected, snorted, taken in pill form, chewed and even ingested in the form of a drink. Synthetic stimulants are becoming increasingly popular as users attempt to alter the chemicals in drugs to create different reactions, and ultimately steer clear of jail time, legal penalties and detection in drug screening efforts.

If a substance is used over a long period of time and the user becomes dependent upon it, a substance abuse issue begins to appear. Substance abuse may lead to substance dependence and with time, addiction. Both mental and physiological dependence requires the development of tolerance leading to withdrawal symptoms. Stimulants come in a very large variety of subtypes and among the most common are caffeine, nicotine, cocaine, methamphetamine, amphetamines, amphetamine congeners, electronic cigarettes, diet pills, plant stimulants, energy drinks, and the ever-evolving designer stimulants such as bath salts. Caffeine and nicotine are the most popular stimulants used today, with roughly 400 million cups of coffee consumed daily and 36.5 million current cigarette smokers, according to a 2015 study conducted by the Centers for Disease Control and Prevention. Nicotine, however, is treated separately psychiatrically under tobacco use disorder.

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.

Opioids, because of their effect on the part of the brain that regulates breathing, can during overdoses lead to the person not breathing (respiratory depression) and therefore result in death. Opiate overdose symptoms and signs can be referred to as the "opioid overdose triad": decreased level of consciousness, pinpoint pupils and respiratory depression. Other symptoms include seizures and muscle spasms. Sometimes a person experiencing an opiate overdose can lead to such a decreased level of consciousness that he or she won't even wake up to their name being called or being shaken by another person.

Prolonged hypoxia from respiratory depression can also lead to detrimental damage to the brain and spinal cord and can leave the person unable to walk or function normally, even if treatment with naloxone is given.

Alcohol also causes respiratory depression and therefore when taken with opioids can increase the risk of respiratory depression and death.

Stimulant use disorder is a type of substance use disorder that involves the abuse of stimulants. It is defined in the DSM-5 as "the continued use of amphetamine-type substances, cocaine, or other stimulants leading to clinically significant impairment or distress, from mild to severe." These psychoactive drugs, known as stimulants, are the most widely used drugs in the world today. Approximately 200 million Americans have used some type of a stimulant in the past year alone.

The more recently published DSM-5 combined substance abuse and substance dependence into a single continuum; this is simply known as substance use disorder and requires more presenting symptoms before a diagnosis is made. It also considers each different substance as its own separate disorder, based upon the same basic criteria. It also distinguishes the difference between dependence and addictions as two separate disorders, not to be confused.

Risk factors for opioid overdose include opioid dependence, injecting opioids, using high doses of opioids, and use together with alcohol or benzodiazepines. The risk is particularly high following detoxification. Dependence on prescription opioids can occur from their use to treat chronic pain.

Examples (and ICD-10 code) include:

- F10.0 alcohol intoxication

- F11.0 opioid intoxication

- F12.0 cannabinoid intoxication

- F13.0 sedative and hypnotic intoxication (see benzodiazepine overdose and barbiturate overdose)

- F14.0 cocaine intoxication

- F15.0 caffeine intoxication

- F16.0 hallucinogen intoxication (See for example Lysergic acid diethylamide effects)

- F17.0 tobacco intoxication

The term contact high is sometimes used to describe intoxication without direct administration, either by second-hand smoke as with cannabis, or by placebo in the presence of others who are high.

A withdrawal syndrome, also called a discontinuation syndrome is a set of symptoms occurring in discontinuation or dosage reduction of some types of medications. The risk of a discontinuation syndrome occurring increases with dosage and length of use.

- Alcohol withdrawal syndrome, symptoms seen when an individual reduces or stops alcohol consumption after periods of excessive alcohol intake

- Antidepressant discontinuation syndrome, a syndrome that can occur following the interruption, dose reduction, or discontinuation of SSRI or SNRI medications

- Antipsychotic withdrawal syndrome or dopamine supersensitivity psychosis, symptoms seen when an individual reduces or suddenly stops antipsychotics

- Benzodiazepine withdrawal syndrome, symptoms that appear when a long term user stops taking benzodiazepines or reduces the dosage

- Cannabis withdrawal, a form of withdrawal associated with the substance cannabis

- Drug withdrawal

- Neonatal withdrawal, a withdrawal syndrome of infants, caused by administration of drugs or the prenatal exposure to a substance

- Nicotine withdrawal, the effects felt by a person who is nicotine dependent and suddenly stops or significantly reduces his or her nicotine intake

- Opioid withdrawal, symptoms seen cessation or rapid reduction of intake of opioid class drugs

Individuals with an alcohol use disorder will often complain of difficulty with interpersonal relationships, problems at work or school, and legal problems. Additionally, people may complain of irritability and insomnia. Alcohol abuse is also an important cause of chronic fatigue.

Signs of alcohol abuse are related to alcohol's effects on organ systems. However, while these findings are often present, they are not necessary to make a diagnosis of alcohol abuse. Signs of alcohol abuse show its drastic effects on the central nervous system, including inebriation and poor judgment; chronic anxiety, irritability, and insomnia. Alcohol's effects on the liver include elevated liver function tests (classically AST is at least twice as high as ALT). Prolonged use leads to cirrhosis and liver failure. With cirrhosis, patients develop an inability to process hormones and toxins. The skin of a patient with alcoholic cirrhosis can feature cherry angiomas, palmar erythema and — in acute liver failure — jaundice and ascites. The derangements of the endocrine system lead to the enlargement of the male breasts. The inability to process toxins leads to liver disease, such as hepatic encephalopathy.

Alcohol abuse can result in brain damage which causes impairments in executive functioning such as impairments to working memory, visuospatial skills, and can cause an abnormal personality as well as affective disorders to develop. Binge drinking is associated with individuals reporting fair to poor health compared to non-binge drinking individuals and which may progressively worsen over time. Alcohol also causes impairment in a person's critical thinking. A person's ability to reason in stressful situations is compromised, and they seem very inattentive to what is going on around them. Social skills are significantly impaired in people suffering from alcoholism due to the neurotoxic effects of alcohol on the brain, especially the prefrontal cortex area of the brain. The social skills that are impaired by alcohol abuse include impairments in perceiving facial emotions, difficulty with perceiving vocal emotions and theory of mind deficits; the ability to understand humour is also impaired in alcohol abusers. Adolescent binge drinkers are most sensitive to damaging neurocognitive functions especially executive functions and memory. People who abuse alcohol are less likely to survive critical illness with a higher risk for having sepsis and were more likely to die during hospitalization.

Alcohol abuse is a previous psychiatric diagnosis in which there is recurring harmful use of alcohol despite its negative consequences. In 2013 it was reclassified as alcohol use disorder (alcoholism) along with alcohol dependence. There are two types of alcohol abuse, those who have anti-social and pleasure-seeking tendencies, and those who are anxiety-ridden people who are able to go without drinking for long periods of time but are unable to control themselves once they start. Binge drinking is another form of alcohol abuse. According to surveys, the heaviest drinkers are the United Kingdom's adolescents. In 2013, 139,000 deaths globally were directly due to alcohol abuse and an additional 384,000 to cirrhosis from excess alcohol consumption.

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.

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.

Combined drug intoxication (CDI), also known as multiple drug intake (MDI) or lethal polydrug/polypharmacy intoxication, is an unnatural cause of human death. CDI is often confused with drug overdose, but it is a completely different phenomenon. It is distinct in that it is due to the simultaneous use of multiple drugs, whether the drugs are prescription, over-the-counter, recreational, or some other combination. Alcohol can exacerbate the symptoms and may directly contribute to increased severity of symptoms. The reasons for toxicity vary depending on the mixture of drugs. Usually, most victims die after using two or more drugs in combination that suppress breathing, and the low blood oxygen level causes brain death.

The CDI/MDI phenomenon seems to be becoming more common in recent years. In December 2007, according to Dr. John Mendelson, a pharmacologist at the California Pacific Medical Center Research Institute, deaths by combined drug intoxication were relatively "rare" ("one in several million"), though they appeared then to be "on the rise". In July 2008, the Associated Press and CNN reported on a medical study showing that over two decades, from 1983 to 2004, such deaths have soared. It has also become a prevalent risk for older patients.