A multitude of reactions can occur during a psychedelic crisis. Some users may experience a general sense of fear or an anxiety attack. A user may be overwhelmed with the disconnection many psychedelics cause, and fear that they are going insane or will never return to reality. This can cause the user to fall into a depressive mood. Other reactions include an amplification of nameless fears; that is, fears that are unfounded and are usually not encountered in normality.

A person having a bad trip might try to harm themselves or others around them. This harm could take the form of suicidal ideation, or full-blown suicide attempts. Because of the magnification of emotions many psychedelics could possibly cause thoughts of death and intensely adverse reactions in some users. Users can believe that their death is imminent or that the very universe itself is collapsing. Rapidly accelerated aging of other people may be experienced, perpetuating the aforementioned fears to an even greater degree.

Some users may experience disorientation. The normal views of time, space and person can be substantially altered, causing fear. Some can worsen their condition by trying to fight the psychedelic experience after embarkment. There can be illusions of insects crawling over or into one's self, or of being in dirty places such as sewers. Some users may experience losing the control of their mind in form of racing thoughts.

In rare cases, an apparent complete loss of control can be observed, and the behavior a person tends to exhibit reflects a lack of normal understanding about navigation of a physical environment. This can cause accidental harm to themselves and others, including running into traffic, flailing around, or, as it may not necessarily involve physical movement, inducing a catatonic state, or causing seemingly random vocalizations, not limited to speech. This may be caused by a failure to recognize external stimuli for what it is. Stanislav Grof explains this feature:

A bad trip (drug-induced temporary psychosis or psychedelic crisis) is a disturbing experience typically associated with use of one or more of various hallucinogens of types including psychedelics, such as LSD, mescaline, psilocybin, and DMT, dissociatives, such as dextromethorphan, and phencyclidine, and deliriants, such as Salvinorin A (the active chemical of "Salvia divinorum").

The manifestations can range from feelings of vague anxiety and alienation to profoundly disturbing states of unrelieved terror, ultimate entrapment, or complete loss of self-identity. Psychedelic specialists in the therapeutic community do not necessarily consider unpleasant experiences as threatening or negative, instead focusing on their potential to greatly benefit the user when properly resolved. Bad trips can be exacerbated by the inexperience or irresponsibility of the user or the lack of proper preparation and environment for the trip, and are reflective of unresolved psychological tensions triggered during the course of the experience.

It is suggested that, at a minimum, such crises be managed by preventing the individual from harming oneself or others by whatever means necessary up to and including physical restraint, providing the patient with a safe and comfortable space, and supervising the intake until all effects of the drug have completely worn off.

An alcohol hangover is associated with a variety of symptoms that may include drowsiness, headache, concentration problems, dry mouth, dizziness, gastrointestinal complaints, fatigue, sweating, nausea, hyper-excitability, anxiety, and a feeling of general discomfort that may last more than 24 hours. Alcohol hangover symptoms develop when blood alcohol concentration falls considerably and peak when it returns to almost zero. Hangover symptoms validated in controlled studies include general malaise, thirst, headache, feeling dizzy or faint, tiredness, loss of appetite, nausea, stomach ache, and feeling as though one's heart is racing. Some symptoms such as changes in sleep pattern and gastrointestinal distress are attributed to direct effects of the alcohol intoxication, or withdrawal symptoms. Drowsiness and impaired cognitive function are the two dominant features of alcohol hangover.

A hangover is the experience of various unpleasant physiological and psychological effects following the consumption of ethanol, as found in wine, beer and distilled spirits. Hangovers can last for several hours or for more than 24 hours. Typical symptoms of a hangover may include headache, drowsiness, concentration problems, dry mouth, dizziness, fatigue, gastrointestinal distress (e.g., vomiting), absence of hunger, depression, sweating, nausea, hyper-excitability and anxiety.

While the causes of a hangover are still poorly understood, several factors are known to be involved including acetaldehyde accumulation, changes in the immune system and glucose metabolism, dehydration, metabolic acidosis, disturbed prostaglandin synthesis, increased cardiac output, vasodilation, sleep deprivation and malnutrition. Beverage-specific effects of additives or by-products such as congeners in alcoholic beverages also play an important role. The symptoms occur typically after the intoxicating effect of the alcohol begins to wear off, generally the morning after a night of heavy drinking.

Though many possible remedies and folk cures have been suggested, there is no compelling evidence to suggest that any are effective for preventing or treating alcohol hangover. Avoiding alcohol or drinking in moderation are the most effective ways to avoid a hangover.

The socioeconomic consequences and health risks of alcohol hangover include workplace absenteeism, impaired job performance, reduced productivity and poor academic achievement. A hangover may also compromise potentially dangerous daily activities such as driving a car or operating heavy machinery.

Acute alcohol poisoning is a related medical term used to indicate a dangerously high concentration of alcohol in the blood, high enough to induce coma, respiratory depression, or even death. It is considered a medical emergency. The term is mostly used by healthcare providers. Toxicologists use the term "alcohol intoxication" to discriminate between alcohol and other toxins.

The signs and symptoms of acute alcohol poisoning include:

- severe confusion, unpredictable behavior and stupor

- sudden lapses into and out of unconsciousness or semi-consciousness (with later alcoholic amnesia)

- vomiting while unconscious or semi-conscious

- seizures

- respiratory depression (fewer than eight breaths a minute)

- pale, bluish, cold and clammy skin due to insufficient oxygen

Alcohol intoxication, also known as drunkenness among other names, is a physiological condition that may result in psychological alterations of consciousness. Drunkenness is induced by the ingestion or consumption of alcohol in a living body. Alcohol intoxication is the result of alcohol entering the bloodstream faster than it can be metabolized by the body. Metabolism results in breaking down the ethanol into non-intoxicating byproducts.

Some effects of alcohol intoxication, such as euphoria and lowered social inhibition, are central to alcohol's desirability as a beverage and its history as one of the world's most widespread recreational drugs. Despite this widespread use and alcohol's legality in most countries, many medical sources tend to describe any level of alcohol intoxication as a form of poisoning due to ethanol's damaging effects on the body in large doses. Some religions consider alcohol intoxication to be a sin.

Symptoms of alcohol intoxication include euphoria, flushed skin, and decreased social inhibition at lower doses, with larger doses producing progressively severe impairments of balance, and decision-making ability as well as nausea or vomiting from alcohol's disruptive effect on the semicircular canals of the inner ear and chemical irritation of the gastric mucosa.

Sufficiently extreme levels of blood-borne alcohol may result in coma or death.

Pregaming (also pre-drinking or pre-loading) is the process of getting drunk prior to going out socializing, typically done by college students and young adults in the United States and Europe, in a manner as cost-efficient as possible, with hard liquor and cheap beer consumed while in small groups.

Although pregaming is typically done before a night out, it can also precede other activities, like attending a college football game, large party, social function, or another activity where possession of alcohol may be limited or prohibited. The name "pregaming" spread from the drinking that took place during tailgating before football games to encompass similar drinking periods.

Other terms for the practice are pre-partying, prinking and (in Europe) prefunking.

The signs and symptoms for excited delirium may include:

- Paranoia

- Disorientation

- Dissociation

- Hyper-aggression

- Tachycardia

- Hallucination

- Diaphoresis

- Incoherent speech or shouting

- Seemingly superhuman strength or endurance (typically while trying to resist restraint)

- Hyperthermia (overheating)/profuse sweating (even in cold weather)

- Inappropriately clothed e.g. having removed garments

Ideas of reference and delusions of reference describe the phenomenon of an individual's experiencing innocuous events or mere coincidences and believing they have strong personal significance. It is "the notion that everything one perceives in the world relates to one's own destiny".

In psychiatry, delusions of reference form part of the diagnostic criteria for psychotic illnesses such as schizophrenia, delusional disorder, bipolar disorder (during the elevated stages of mania), as well as schizotypal personality disorder. To a lesser extent, it can be a hallmark of paranoid personality disorder. Such symptoms can also be caused by intoxication, especially with hallucinogens or stimulants like methamphetamine.

Excited delirium, also known as agitated delirium, is a condition that presents with psychomotor agitation, delirium, and sweating. It may include attempts at violence, unexpected strength, and very high body temperature. Complications may include rhabdomyolysis or high blood potassium.

The cause is often related to long term drug use or mental illness. Commonly involved drugs include cocaine, methamphetamine, or certain substituted cathinones. In those with mental illness, rapidly stopping medications such as antipsychotics may trigger the condition. The underlying mechanism is believed to involve dysfunction of the dopamine system in the brain. The diagnosis is recognized by the American College of Emergency Physicians but is not in the Diagnostic and Statistical Manual of Mental Disorders or the International Classification of Diseases.

Treatment initially includes medications to sedate the person such as ketamine or midazolam and haloperidol injected into a muscle. Rapid cooling may be required in those with high body temperature. Other supportive measures such as intravenous fluids and sodium bicarbonate may be useful. The risk of death among those affected is less than 10%. If death occurs it is typically sudden and cardiac in nature.

How frequently cases occur is unknown. Those who die from the condition are typically male with an average age of 36. Often law enforcement has used tasers or physical measures in these cases. A similar condition was described in the 1800s and was referred to as Bell's mania. The term "excited delirium" did not come into use until the 1980s.

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.

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.

People who engage in polypharmacy and other hypochondriac behaviors are at an elevated risk of death from CDI. Elderly people are at the highest risk of CDI, because of having many age-related health problems requiring many medications combined with age-impaired judgment, leading to confusion in taking medications.

A drug-related blackout is a phenomenon caused by the intake of any substance or medication in which short term and long term memory creation is impaired, therefore causing a complete inability to recall the past. Blackouts are most frequently associated with GABAergic drugs. Blackouts are frequently described as having effects similar to that of anterograde amnesia, in which the subject cannot recall any events after the event that caused amnesia. Research on alcohol blackouts was begun by E. M. Jellinek in the 1940s. Using data from a survey of Alcoholics Anonymous (AA) members, he came to believe that blackouts would be a good determinant of alcoholism. However, there are conflicting views whether this is true. The negative psychological effects of an alcohol-related blackout are often worsened by those who suffer from anxiety disorders. Impairment of the liver will also allow more alcohol to reach the brain and hasten the individual's blackout.

The term "blackout" can also refer to a complete loss of consciousness, or syncope.

Both alcoholic hallucinosis and DTs have been thought of as different manifestations of the same physiological process in the body during alcohol withdrawal. Alcoholic hallucinosis is a much less serious diagnosis than delirium tremens. Delirium tremens (DTs) do not appear suddenly, unlike alcoholic hallucinosis. DTs also take approximately 48 to 72 hours to appear after the heavy drinking stops. A tremor develops in the hands and can also affect the head and body. A common symptom of delirium tremens is that people become severely uncoordinated. The biggest difference between alcoholic hallucinosis and delirium tremens is that alcoholic hallucinosis have a much better prognosis than DTs. Moreover, delirium tremens can be fatal when untreated.

Different concentrations of alcohol in the human body have different effects on the subject.

The following lists the common effects of alcohol on the body, depending on the blood alcohol concentration (BAC). However, tolerance varies considerably between individuals, as does individual response to a given dosage; the effects of alcohol differ widely between people. Hence, BAC percentages are just estimates used for illustrative purposes.

- Euphoria (BAC = 0.03% to 0.12%):

- Overall improvement in mood and possible euphoria

- Increased self-confidence

- Increased sociability

- Decreased anxiety

- Shortened attention span

- Flushed appearance

- Impaired judgment

- Impaired fine muscle coordination

- Lethargy (BAC = 0.09% to 0.25%)

- Sedation

- Impaired memory and comprehension

- Delayed reactions

- Ataxia; balance difficulty; unbalanced walk

- Blurred vision; other senses may be impaired

- Confusion (BAC = 0.18% to 0.30%)

- Profound confusion

- Impaired senses

- Analgesia

- Increased ataxia; impaired speech; staggering

- Dizziness often associated with nausea ("the spins")

- Vomiting (emesis)

- Stupor (BAC = 0.25% to 0.40%)

- Severe ataxia

- Lapses in and out of consciousness

- Unconsciousness

- Anterograde amnesia

- Vomiting (death may occur due to inhalation of vomit (pulmonary aspiration) while unconscious)

- Respiratory depression (potentially life-threatening)

- Decreased heart rate (usually results in coldness and/or numbness of the limbs)

- Urinary incontinence

- Coma (BAC = 0.35% to 0.80%)

- Unconsciousness (coma)

- Depressed reflexes (i.e., pupils do not respond appropriately to changes in light)

- Marked and life-threatening respiratory depression

- Markedly decreased heart rate

- Most deaths from alcohol poisoning are caused by dosage levels in this range.

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.

Substance abuse, also known as drug abuse, is a patterned use of a drug in which the user consumes the substance in amounts or with methods which are harmful to themselves or others, and is a form of substance-related disorder. Widely differing definitions of drug abuse are used in public health, medical and criminal justice contexts. In some cases criminal or anti-social behavior occurs when the person is under the influence of a drug, and long term personality changes in individuals may occur as well. In addition to possible physical, social, and psychological harm, use of some drugs may also lead to criminal penalties, although these vary widely depending on the local jurisdiction.

Drugs most often associated with this term include: alcohol, cannabis, barbiturates, benzodiazepines, cocaine, methaqualone, opioids and some substituted amphetamines. The exact cause of substance abuse is not clear, with the two predominant theories being: either a genetic disposition which is learned from others, or a habit which if addiction develops, manifests itself as a chronic debilitating disease.

In 2010 about 5% of people (230 million) used an illicit substance. Of these 27 million have high-risk drug use otherwise known as recurrent drug use causing harm to their health, psychological problems, or social problems or puts them at risk of those dangers. In 2015 substance use disorders resulted in 307,400 deaths, up from 165,000 deaths in 1990. Of these, the highest numbers are from alcohol use disorders at 137,500, opioid use disorders at 122,100 deaths, amphetamine use disorders at 12,200 deaths, and cocaine use disorders at 11,100.

The cause of alcoholic hallucinosis is unclear. It seems to be highly related to the presence of dopamine in the limbic system with the possibility of other systems. There are many symptoms that could possibly occur before the hallucinations begin. Symptoms include headache, dizziness, irritability, insomnia, and indisposition. Typically, alcoholic hallucinosis has a sudden onset.

Depression is a state of a low mood and aversion to activity that can affect a person's thoughts, behavior, feelings, and sense of well-being. A depressed mood is a normal temporary reaction to life events such as loss of a loved one. It is also a symptom of some physical diseases and a side effect of some drugs and medical treatments. Depressed mood is also a symptom of some mood disorders such as major depressive disorder or dysthymia.

People with a depressed mood may be notably sad, anxious, or empty; they may also feel notably hopeless, helpless, dejected, or worthless. Other symptoms expressed may include senses of guilt, irritability, or anger. Further feelings expressed by these individuals may include feeling ashamed or an expressed restlessness. These individuals may notably lose interest in activities that they once considered pleasurable to family and friends or otherwise experience either a loss of appetite or overeating. Experiencing problems concentrating, remembering general facts or details, otherwise making decisions or experiencing relationship difficulties may also be notable factors in these individuals' depression and may also lead to their attempting or actually dying by suicide.

Expressed insomnia, excessive sleeping, fatigue, and vocalizing general aches, pains, and digestive problems and a reduced energy may also be present in individuals experiencing depression.

An alcohol enema, also known colloquially as butt-chugging, is the act of introducing alcohol into the rectum and colon via the anus. This method of alcohol consumption can be dangerous and even deadly because it leads to faster intoxication since the alcohol is absorbed directly into the bloodstream and neutralizes the body's ability to reject the toxin by vomiting.

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.

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.

The short-term effects of alcohol (also known formally as ethanol) consumption–due to drinking beer, wine, distilled spirits or other alcoholic beverages–range from a decrease in anxiety and motor skills and euphoria at lower doses to intoxication (drunkenness), stupor, unconsciousness, anterograde amnesia (memory "blackouts"), and central nervous system depression at higher doses. Cell membranes are highly permeable to alcohol, so once alcohol is in the bloodstream it can diffuse into nearly every cell in the body.

The concentration of alcohol in blood is measured via blood alcohol content (BAC). The amount and circumstances of consumption play a large part in determining the extent of intoxication; for example, eating a heavy meal before alcohol consumption causes alcohol to absorb more slowly. The amount of alcohol consumed largely determines the extent of hangovers, although hydration also plays a role. After excessive drinking, stupor and unconsciousness can occur. Extreme levels of consumption can lead to alcohol poisoning and death (a concentration in the blood stream of 0.40% will kill half of those affected). Alcohol may also cause death indirectly, by asphyxiation from vomit.

Alcohol can greatly exacerbate sleep problems. During abstinence, residual disruptions in sleep regularity and sleep patterns are the greatest predictors of relapse.