A blackout is a phenomenon caused by the intake of alcohol

Alcoholic beverage

An alcoholic beverage is a drink containing ethanol, commonly known as alcohol. Alcoholic beverages are divided into three general classes: beers, wines, and spirits. They are legally consumed in most countries, and over 100 countries have laws regulating their production, sale, and consumption...

 or other substance in which long term memory creation is impaired or there is a complete inability to recall the past. Blackouts are frequently described as having effects similar to that of anterograde amnesia

Anterograde amnesia

Anterograde amnesia is a loss of the ability to create new memories after the event that caused the amnesia, leading to a partial or complete inability to recall the recent past, while long-term memories from before the event remain intact. This is in contrast to retrograde amnesia, where memories...

, in which the subject cannot create memories after the event that caused amnesia

Amnesia

Amnesia is a condition in which one's memory is lost. The causes of amnesia have traditionally been divided into categories. Memory appears to be stored in several parts of the limbic system of the brain, and any condition that interferes with the function of this system can cause amnesia...

. 'Blacking out' is not to be confused with the mutually exclusive

Mutually exclusive

In layman's terms, two events are mutually exclusive if they cannot occur at the same time. An example is tossing a coin once, which can result in either heads or tails, but not both....

 act of 'passing out', which means loss of consciousness. Research on alcohol blackouts was begun by E. M. Jellinek in the 1940s. Using data from a survey of Alcoholics Anonymous

Alcoholics Anonymous

Alcoholics Anonymous is an international mutual aid movement which says its "primary purpose is to stay sober and help other alcoholics achieve sobriety." Now claiming more than 2 million members, AA was founded in 1935 by Bill Wilson and Dr. Bob Smith in Akron, Ohio...

 (AA) members, he came to believe that blackouts would be a good predictor of alcoholism. However, there are conflicting views as to whether this is true. The negative psychological effects of an alcohol related blackout are often worsened by those who suffer from anxiety disorders.

Alcohol and long-term memory

Various studies have also proven links between general alcohol consumption and its effects on memory creation. Particularly, these studies have shown that associations made between words and objects when intoxicated are less easily recalled than associations made when not intoxicated. Later blackout-specific studies have indicated that alcohol specifically impairs the brain's ability to take short-term memories and experiences and transfer them to long-term memory.

It is a common misconception that blackouts generally occur only to alcoholics; research suggests that binge drinkers, such as college students, are often at risk as well. In a 2002 survey of college students by researchers at Duke University Medical Center, 40% of those surveyed who had consumed alcohol recently reported having experienced a blackout within the preceding year.

Types of blackouts

Blackouts can generally be divided into two categories, "en bloc" blackouts, and "fragmentary" blackouts. En bloc blackouts are classified by the inability to later recall any memories from the intoxicated period, even when prompted. These blackouts are characterized also by the ability to easily recall things that have occurred within the last 2 minutes, yet inability to recall anything prior to this period. As such, a person experiencing an en bloc blackout may not appear to be doing so, as they can carry on conversations or even manage to accomplish difficult feats. It is difficult to determine the end of this type of blackout as sleep typically occurs before they end. Fragmentary blackouts are characterized by the ability to recall certain events from an intoxicated period, yet be unaware that other memories are missing until reminded of the existence of these 'gaps' in memory. This phenomenon is also termed a brownout. Research indicates that fragmentary blackouts, or brownouts are far more common than en bloc blackouts.

Causes

Blackouts are commonly associated with the consumption of large amounts of alcohol; however, surveys of drinkers experiencing blackouts have indicated that they are not directly related to the amount of alcohol consumed. Respondents reported they frequently recalled having "drunk as much or more without memory loss", compared to instances of blacking out. Subsequent research has indicated that blackouts are most likely caused by a rapid increase in a person's blood-alcohol concentration

Blood alcohol content

Blood alcohol content , also called blood alcohol concentration, blood ethanol concentration, or blood alcohol level is most commonly used as a metric of alcohol intoxication for legal or medical purposes....

. One study, in particular, resulted in subjects being stratified easily into two groups, those who consumed alcohol very quickly, and blacked out, and those who did not black out by drinking alcohol slowly, despite being extremely intoxicated by the end of the study.

Other GABA-A Agonist Drugs

Benzodiazepines such as flunitrazepam

Flunitrazepam

Flunitrazepam is marketed as a potent hypnotic, sedative, anticonvulsant, anxiolytic, amnestic, and skeletal muscle relaxant drug most commonly known as Rohypnol...

, midazolam

Midazolam

Midazolam is a short-acting drug in the benzodiazepine class developed by Hoffmann-La Roche in the 1970s. The drug is used for treatment of acute seizures, moderate to severe insomnia, and for inducing sedation and amnesia before medical procedures. It possesses profoundly potent anxiolytic,...

, and alprazolam

Alprazolam

Alprazolam is a short-acting anxiolytic of the benzodiazepine class of psychoactive drugs. Alprazolam, like other benzodiazepines, binds to specific sites on the GABAA gamma-amino-butyric acid receptor...

, and barbiturates such as phenobarbital

Phenobarbital

Phenobarbital or phenobarbitone is a barbiturate, first marketed as Luminal by Friedr. Bayer et comp. It is the most widely used anticonvulsant worldwide, and the oldest still commonly used. It also has sedative and hypnotic properties but, as with other barbiturates, has been superseded by the...

, which also act as GABA-A agonists, are known to cause blackouts as a result of high dose use.

Predisposition to blackouts

Research indicates that some users of alcohol, particularly those with a history of blackouts, are predisposed to experience blackouts more frequently than others. One such study indicated a link between prenatal exposure to alcohol and vulnerability towards blackouts, in addition to the oft-cited link between this type of exposure and alcoholism

Alcoholism

Alcoholism is a broad term for problems with alcohol, and is generally used to mean compulsive and uncontrolled consumption of alcoholic beverages, usually to the detriment of the drinker's health, personal relationships, and social standing...

. Alternatively, another study has indicated that there appears to be a genetic predisposition towards blacking out, suggesting that some individuals are made to be susceptible to alcohol related amnesia.