There are a number of factors that play a part in whether or not you will get a hangover after a night of drinking. Generally speaking, the more alcohol you consume, the greater the chances of a hangover. Other factors in development of a hangover include:

• The ethanol contained in alcoholic beverages has a dehydrating effect which causes headaches, dry mouth and tiredness. This effect can be lessened by drinking plenty of water before and throughout your night of drinking.

• Your liver breaks ethanol down with the aid of enzymes produced by liver cells. These chemical reactions do many things including impairing the liver's ability to supply glucose to tissues, in particular to the brain. Glucose is responsible for the brain's energy and the lack thereof results in fatigue, weakness, moodiness and decreased attention.

• Congeners are the by-products of the process of alcohol fermentation and exaggerate the symptoms of a hangover. The more congeners consumed, the worse a hangover is likely to be. Dark spirits such as brandy, whiskey and red wine contain more congeners than lighter spirits like vodka and white wine. Likewise cheaper spirits have had fewer of these impurities removed and are more likely to cause a hangover.

• Some people believe the sugar in sweeter cocktails contribute to the severity of hangovers.

• Smokers and even some non-smokers tend to smoke more when they are drinking and this can lead to nicotine poisoning which will also worsen hangovers.

• Some people are genetically lucky when it comes to hangovers and rarely, if ever, suffer the effects.  

• Weight is a factor. The less one weighs the more that person will feel the effects and after effects of alcohol.

• The older you are the more likely you are to have a severe hangover. This is usually not a factor because we tend to take it a little easier as we learn from the mistakes of the past. But you have been warned.

• Finally, the more you drink or the more you guzzle in a short time span, the worse your hangover will be.

(from About.com http://cocktails.about.com/od/healthsafety/a/hngovr_101a_2.htm).

Alcohol has an irritating effect on the lining of the stomach which can produce nausea. When alcohol is consumed, the liver has the job of trying to eliminate this toxin from the body. Through a series of enzymes alcohol is converted to acetaldehyde, and then to acetic acid. It takes a while for the body to break down all the acetaldehyde (which is more toxic than the alcohol itself). Drinking large quantities of alcohol can cause significant buildup of acetaldehyde before the liver can break it all down into acetic acid. Some people lack the enzyme acetaldehyde dehydrogenase, and experience hangover-like symptoms and other very unpleasant symptoms immediately after consuming alcohol. In layman's terms, the individual is "allergic to alcohol." The inability to break down the alcohol is a genetically transferred trait, and is most often seen in Asians. This same extremely unpleasant reaction to alcohol can be induced by taking the drug disulfiram (Antabuse). Disulfiram inhibits the ability of acetaldehyde dehydrogenase to break acetaldehyde. Alcohol consumption can also deplete the liver's supply of glutathione and other reductive detoxification agent. This further reduces the body's ability to remove acetaldehyde and other toxins. As mentioned above, congeners can be a large factor in causing hangover. These include other alcohols that are byproducts of alcoholic fermentation or metals added to sweet liqueurs to enhance their flavor. Other byproducts from the process of fermentation of alcohol contribute to the unique tastes, color and aroma of the different liquors. Darker liquors like bourbon or scotch usually have more congeners than clear liquors like vodka. Red wine has more congeners that white wine. Some people have a strong negative reaction to red wine, distinct from a hangover, called red wine headache that can occur within 15 minutes of drinking a single glass of red wine. Symptoms generally include headache accompanied by nausea and flushing.