A hangover (veisalgia) describes the sum of unpleasant physiological effects following heavy consumption of drugs, particularly alcoholic beverages. The most commonly reported characteristics of a hangover include headache, nausea, sensitivity to light and noise, lethargy, dysphoria, and thirst.

Hypoglycemia, dehydration, acetaldehyde intoxication, and vitamin B12 deficiency are all theorized causes of hangover symptoms. Hangovers may last up to two or three days after alcohol was last consumed. Roughly 25-30% of drinkers are resistant to hangover symptoms. Some aspects of a hangover can be viewed as symptoms of acute ethanol withdrawal, similar to the longer-duration effects of withdrawal from alcoholism, as determined by studying the increases in brain reward thresholds in rats (the amount of current required to receive to electrodes implanted in the lateral hypothalamus) following ethanol injection.

Symptoms

An alcohol hangover is associated with a variety of symptoms that may include dehydration, fatigue, headache, nausea, vomiting, diarrhoea, weakness, elevated body temperature, hypersalivation, difficulty concentrating, anxiety, irritability, sensitivity to light and noise, erratic motor functions, trouble sleeping, and lack of depth perception. Many people will also be repulsed by the thought or taste of alcohol during a hangover. The symptoms vary from person to person, and occasion to occasion, usually beginning several hours after drinking. It is not clear whether hangovers directly affect cognitive abilities. Fact: date=May 2008 In some rare cases, these symptoms can be additive to the point of hospitalization.

Causes

Ethanol has a dehydrating effect by causing increased urine production (such substances are known as diuretics), which causes headaches, dry mouth, and lethargy. This can be mitigated by drinking water or an oral electrolyte solution after consumption of alcohol. Alcohol's effect on the stomach lining can account for nausea. Because of the increased NADH production during metabolism of ethanol by the enzymes alcohol dehydrogenase and aldehyde dehydrogenase, excess NADH can build up and slow down gluconeogenesis in the liver, thus causing hypoglycemia.

Another factor contributing to a hangover are the products from the breakdown of ethanol via liver enzymes. Ethanol is converted to acetaldehyde by the enzyme alcohol dehydrogenase, and then from acetaldehyde to acetic acid by the enzyme acetaldehyde dehydrogenase. Acetaldehyde (ethanal) is between 10 and 30 times more toxic than alcohol itself, as well as being carcinogenic and mutagenic.

These two reactions also require the conversion of NAD+ to NADH. With an excess of NADH, the lactate dehydrogenase reaction is driven to produce lactate from pyruvate (the end product of glycolysis) in order to regenerate NAD+ and sustain life. This diverts pyruvate from other pathways such as gluconeogenesis, thereby impairing the ability of the liver to supply glucose to tissues, especially the brain. Because glucose is the primary energy source of the brain, this lack of glucose contributes to hangover symptoms such as fatigue, weakness, mood disturbances, and decreased attention and concentration.