The exact cause of hangovers is not completely understood, but they are a well known problem with ingesting alcohol. Bad alcohol hangovers are considered to be worse than the day-after effects of nearly any other psychoactive. Primary symptoms include:

• Headache
• Irritability, bad mood
• Thirst
• Nausea, vomiting, and/or dry-heaves
• Vertigo (dizziness that becomes worse with movement)
• Light and sound sensitivity (loud noises and bright lights cause pain/discomfort)
• Inability to think clearly
• Muscle fatigue and pain
• Sweating, tremors

There are a variety of factors that determine how bad a hangover is. The primary causes of hangover are believed to be dehydration and related electrolyte imbalance, blood sugar regulation disturbance, acute withdrawal, toxicity from alcohol metabolites, interaction with congeners (non-alcohol components of drinks), reduced sleep quality, and personal biological profile.

Dehydration and Electrolyte Imbalance : Alcohol is a diueretic, causing the body to urinate more than normal, leading to dehydration and electrolyte imbalances. Alcohol intake inhibits 'anti-diueretic hormone' (ADH, also known as vasopressin), which alters how urine is produced. Reduced ADH levels cause more urine to be produced and electrolytes (salts such as sodium, potassium, or magnesium) are expelled with the urine. Generally, dehydration and electrolyte imbalances just make everything else worse and even moderate loss of fluid can cause dizziness, lightheadedness, weakness, and difficulty thinking clearly.

Blood Sugar / Hypoglycemia : Alcohol ingestion can also cause changes to blood sugar levels, tending to cause hypoglycemia (low blood sugar) that can contribute to weakness, fatigue and bad mood.

Sleep Quality : It is common for people to drink heavily just before or during their normal sleep period. Drinking heavily worsens sleep quality, and reduced sleep quality contributes to hangover-related tiredness and generally worsens other symptoms.

Short-term tolerance and withdrawal : Perhaps the most surprising component of alcohol hangovers is that it is currently believed that alcohol causes short-term tolerance followed by acute withdrawal as blood levels fall. This short-term accustomation to the presence of alcohol may lead to withdrawal effects as the body re-calibrates as the alcohol is cleared from the system. This 'acute withdrawal' effect is the reason the "hair of the dog" hangover remedy works at all (drinking more alcohol in the morning to combat a bad hangover). The mechanism for the acute withdrawal symptoms is currently believed to be the short-term down regulation of GABA receptors and up-regulation of glutamate receptors as the body counterbalances the sedative effects of the alcohol. As alcohol levels in the bloodstream fall, it takes time for the GABA and glutamate systems to return to normal.

Alcohol Metabolites : Ethyl alcohol (drinking alcohol) is metabolized first by the enzyme alcohol dehydogenase into acetaldehyde, which is then metabolized into acetate by the enzyme aldehyde dehydrogenase. Although acetaldehyde is quickly metabolized by most people and its exact role in hangover is not well understood, acetaldehyde by itself is toxic at moderate doses, causing sweating, nausea, and vomiting.

Congeners : Congeners (aka "Other Stuff In Alcoholic Drinks") are also believed to play a large role in many hangovers. Congeners include tannins, flavorings, colorings, etc. Red wine, for instance, is known to cause mild histamine reactions in many people whereas white wine does not. The congeners are believed to contribute to hangovers from drinking darker alcoholic beverages such as red wine, whiskey, brandy, etc.

Long-term tolerance : Regular drinkers tend to have less symptoms from hangover than occasional binge drinkers. As with many other toxins, it is likely that those who are regular alcohol users have developed tolerance to and physical ability to manage the toxic effects. Occasional drinkers are more likely to get bad hangovers than regular ones.

Personal biological profiles : Everyone reacts differently to every substance, and alcohol is no exception. Family history, personal idiosyncrasies, and a variety of other poorly understood factors determine whether someone will get a hangover and/or how bad it is. There are many stories of people who get no hangover whatsoever even after extremely heavy binge drinking. It is also common for this 'ability' to change over time, and some people who formerly never got hangovers find themselves getting hangovers as bad as everyone else.