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.

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.

Alcohol-related brain damage is the damage that occurs to brain structures or function of the central nervous system as a result of the direct neurotoxic effects of alcohol intoxication or acute withdrawal. The frontal lobes are the most damaged region of the brains of alcohol abusers but other regions of the brain are also affected. The damage that occurs from heavy drinking/high blood alcohol levels causes impairments in judgement and decision making and social skills. These brain changes are linked to poor behavioural control and impulsivity, which tend to worsen the existing addiction problem.

The problems of alcoholism are well known, such as memory disorders, liver disease, high blood pressure, muscle weakness, heart problems, anaemia, low immune function, disorders of the digestive system and pancreatic problems as well as depression, unemployment and family problems including child abuse. Recently attention has been increasingly focused on binge drinking by adolescents and young adults due to neurochemical changes and brain damage which, unlike with alcoholism, can occur after a relatively short period of time; the damage is particularly evident in the corticolimbic region. This brain damage increases the risk of abnormalities in mood and cognitive abilities, increases the risk of dementia and additionally binge drinkers have an increased risk of developing chronic alcoholism.

Individuals who are impulsive are at high risk of addiction due to impaired behavioural control and increased sensation seeking behaviour. Alcohol abuse, especially during adolescence, causes a deterioration of executive functions in the frontal lobe. This brain damage from alcohol actually increases impulsivity and therefore worsens the addictive disorder.

There are five main stages of alcoholism. The first stage,occasional abuse and binge drinking, in this stage one may want to just experiment with alcohol and test their limits. These drinkers may be new to different forms of alcohol. This experimental stage is commonly seen in teens and young adults. These experimental drinkers also frequently engage in binge drinking. While they may not drink regularly, they consume exceptionally enormous amounts of alcohol at one time.

The second stage, increased drinking, in this stage one will leave the experimental stage and start drinking on a regular basis. Instead of just drinking at parties occasionally, one may find themselves drinking every weekend. Increased alcohol consumption can also lead to drinking for these reasons: as an excuse to get together with friends, to alleviate stress, out of boredom, or to combat sadness or loneliness.

The third stage, problem drinking, one will drink to get rid of their problems for them at any moment. As increased drinking continues, one becomes more dependent on alcohol and are at risk of developing alcoholism.

The fourth stage, alcohol dependence, this forms after the problem drinking stage. At this point, one has an attachment to alcohol that has taken over their regular routine. They are aware of the adverse effects, but no longer have control over their alcohol consumption. Alcohol dependence also means that one has developed a tolerance to drinking. As a result, they may have to consume larger quantities to get “buzzed” or drunk.

The fifth stage, addiction and alcoholism, this is the final and most harmful stage. One is addicted and dependent and must have alcohol all the time, if not they have withdrawals. Alcohol withdrawal is the changes the body goes through when a person suddenly stops drinking after prolonged alcohol abuse, or if one does not have alcohol for a period of time. Symptoms include trembling (shakes), insomnia, anxiety, and other physical and mental symptoms. If the alcohol is withdrawn suddenly, the brain is like an accelerated vehicle that has lost its brakes. Not surprisingly, most symptoms of withdrawal are symptoms that occur when the brain is overstimulated (Drugs.com). People with alcohol addiction physically crave the substance and are often inconsolable until they start drinking again. With prolonged abstinence neurogenesis occurs which can potentially reverse the damage from alcohol abuse.

Typically the severity of the symptoms experienced will depend on the amount and duration of prior alcohol consumption, as well as the number and severity of previous withdrawals. Even the most severe of these symptoms can occur in as little as 2 hours after cessation; therefore, the overall unpredictability necessitates either pre-planned hospitalization, treatment coordinated with a doctor, or at the very least rapid access to medical care, and a supporting system of friends or family should be introduced prior to addressing detoxification. In many cases, however, symptoms follow a reasonably predictable time frame as exampled below:

Six to 12 hours after the ingestion of the last drink, withdrawal symptoms such as shaking, headache, sweating, anxiety, nausea or vomiting occur. Other comparable symptoms may also exist in this period. Twelve to 24 hours after cessation, the condition may progress to such major symptoms as confusion, hallucinations (with awareness of reality), tremor, agitation, and similar ailments.

At 24 to 48 hours following the last ethanol ingestion, the possibility of seizures should be anticipated. Meanwhile, none of the earlier withdrawal symptoms will have abated. Seizures carry the risk of death for the alcoholic.

Although, most often, the patient's condition begins to improve past the 48-hour mark, it can sometimes continue to increase in severity to delirium tremens, characterized by hallucinations that are indistinguishable from reality, severe confusion, more seizures, high blood pressure and fever which can persist anywhere from 4 to 12 days.

Signs and symptoms of alcohol withdrawal occur primarily in the central nervous system. The severity of withdrawal can vary from mild symptoms such as sleep disturbances and anxiety to severe and life-threatening symptoms such as delirium, hallucinations, and autonomic instability.

Withdrawal usually begins 6 to 24 hours after the last drink. It can last for up to one week. To be classified as alcohol withdrawal syndrome, patients must exhibit at least two of the following symptoms: increased hand tremor, insomnia, nausea or vomiting, transient hallucinations (auditory, visual or tactile), psychomotor agitation, anxiety, tonic-clonic seizures, and autonomic instability.

The severity of symptoms is dictated by a number of factors, the most important of which are degree of alcohol intake, length of time the individual has been using alcohol, and previous history of alcohol withdrawal. Symptoms are also grouped together and classified:

- Alcohol hallucinosis: patients have transient visual, auditory, or tactile hallucinations, but are otherwise clear.

- Withdrawal seizures: seizures occur within 48 hours of alcohol cessations and occur either as a single generalized tonic-clonic seizure or as a brief episode of multiple seizures.

- Delirium tremens: hyperadrenergic state, disorientation, tremors, diaphoresis, impaired attention/consciousness, and visual and auditory hallucinations. This usually occurs 24 to 72 hours after alcohol cessation. Delirium tremens is the most severe form of withdrawal and occurs in 5 to 20% of patients experiencing detoxification and 1/3 of patients experiencing withdrawal seizures.

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.

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

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.

Alcohol (also known as ethanol) has a number of effects on health. Short-term effects of alcohol consumption include intoxication and dehydration. Long-term effects of alcohol consumption include changes in the metabolism of the liver and brain and alcoholism. Alcohol intoxication affects the brain, causing slurred speech, clumsiness, and delayed reflexes. Alcohol stimulates insulin production, which speeds up glucose metabolism and can result in low blood sugar, causing irritability and possibly death for diabetics. A 2014 World Health Organization report found that harmful alcohol consumption caused about 3.3 million deaths annually worldwide.

However, some effects of alcohol consumption are beneficial. Although even moderate alcohol consumption increased the risk of death in younger people, it has been shown to "decrease" the risk of death for individuals ages 55+ (due to decreased risk of ischemic heart disease).

The median lethal dose of alcohol in test animals is a blood alcohol content of 0.45%. This is about six times the level of ordinary intoxication (0.08%), but vomiting or unconsciousness may occur much sooner in people who have a low tolerance for alcohol. The high tolerance of chronic heavy drinkers may allow some of them to remain conscious at levels above 0.40%, although serious health hazards are incurred at this level.

Alcohol also limits the production of vasopressin (ADH) from the hypothalamus and the secretion of this hormone from the posterior pituitary gland. This is what causes severe dehydration when alcohol is consumed in large amounts. It also causes a high concentration of water in the urine and vomit and the intense thirst that goes along with a hangover.

Stress, hangovers, and the oral contraceptive pill may increase the desire for alcohol because these things will lower the level of testosterone and alcohol will acutely elevate it. Tobacco has the same effect of increasing the craving for alcohol.

While researchers have found that moderate alcohol consumption in older adults is associated with better cognition and well-being than abstinence, excessive alcohol consumption is associated with widespread and significant brain lesions. The effects can manifest much later—mid-life Alcohol Use Disorder has been found to correlate with increased risk of severe cognitive and memory deficits in later life. Alcohol related brain damage is not only due to the direct toxic effects of alcohol; alcohol withdrawal, nutritional deficiency, electrolyte disturbances, and liver damage are also believed to contribute to alcohol-related brain damage.

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)

College drinking is the consumption of alcohol by students on the campus of any college or university. The age at which it is legal to drink varies by country and affects whether college drinking is considered illegal (e.g., as in the United States, where it is illegal for those under the age of 21 to drink).

Substance intoxication is a type of substance use disorder which is potentially maladaptive and impairing, but reversible, and associated with recent use.

If the symptoms are severe, the term "substance intoxication delirium" may be used.

Generic slang terms include: getting high or being stoned or blazed (all usually in reference to cannabis), with many more specific slang terms for each particular type of intoxicant. Alcohol intoxication is even graded in intensity, from buzzed, to tipsy, all the way up to hammered, smashed, wasted, destroyed, and a number of other similar terms.

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.

Alcohol is a liquid form substance which contains ethyl alcohol (also known formally as ethanol) that can cause harm and even damage to a persons DNA. "Alcohol consumption is recognized worldwide as a leading risk factor for disease, disability, and death." and is rated as the most used and abused substance by adolescences. Adolescence is a transitional stage of physical and psychological changes, usually a time in a person life in which they go through puberty. Combining these transitional stages and the intake of alcohol, can leave a number of consequences for an adolescent.

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.

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 related brain damage causes a wide range of executive function impairments including, impaired judgment, blunted affect, poor insight, social withdrawal, depression, reduced motivation, distractibility, attentional and impulse control deficits.

Ethanol is the type of alcohol found in alcoholic beverages. It is a volatile, flammable, colorless liquid that acts as a central nervous system depressant. Ethanol can impair different types of memory.

Alcohol myopia is a cognitive-physiological theory on alcohol abuse in which many of alcohol's social and stress-reducing effects, which may underlie its addictive capacity, are explained as a consequence of alcohol's narrowing of perceptual and cognitive functioning. The alcohol myopia model posits that rather than disinhibit, alcohol produces a myopia effect that causes users to pay more attention to salient environmental cues and less attention to less salient cues. Therefore, alcohol's myopic effects cause intoxicated people to respond almost exclusively to their immediate environment. This "nearsightedness" limits their ability to consider future consequences of their actions as well as regulate their reactive impulses.

Alcohol's ability to alter behavior and decision-making stems from its impact on synaptic transmission at GABA receptors. Alcohol's effects on the synaptic level dampen the brain's processing ability and limit attentional capacity.

Overall, the alcohol myopia theory proposes that intoxicated individuals will act rashly and will choose overly simple solutions to complex problems.

The short-term effects of alcohol consumption range from a decrease in anxiety and motor skills at lower doses to unconsciousness, anterograde amnesia, 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. Hydration also plays a role, especially in determining the extent of hangovers. After excessive drinking, unconsciousness can occur and 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.

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.

Alcohol abuse among pregnant women causes their baby to develop fetal alcohol syndrome. Fetal alcohol syndrome is the pattern of physical abnormalities and the impairment of mental development which is seen with increasing frequency among children with alcoholic mothers. Alcohol exposure in a developing fetus can result in slowed development of the fetal brain, resulting in severe retardation or death. Surviving infants may suffer severe abnormalities such as abnormal eyes, fissures, lips and incomplete cerebella. Some infants may develop lung disease. It is even possible that the baby throughout pregnancy will develop heart defects such as ventricular septal defect or atrial septal defect. Experts suggest that pregnant women take no more than one unit of alcohol per day. However, other organizations advise complete abstinence from alcohol while pregnant.

Chronic use of alcohol used to induce sleep can lead to insomnia: frequent moving between sleep stages occurs, with awakenings due to headaches and diaphoresis. Stopping chronic alcohol abuse can also lead to profound disturbances of sleep with vivid dreams. Chronic alcohol abuse is associated with NREM stage 3 and 4 sleep as well as suppression of REM sleep and REM sleep fragmentation. During withdrawal REM sleep is typically exaggerated as part of a rebound effect.

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.