Alcohol intolerance is characterized by immediate unpleasant reactions after drinking alcohol. The most common signs and symptoms of alcohol intolerance are stuffy nose and skin flushing. Alcohol intolerance is caused by a genetic condition in which the body is unable to break down alcohol efficiently, usually found in Asians. These individuals accumulate acetaldehyde, the primary metabolite of ethanol, because of a genetic polymorphism of aldehyde dehydrogenase (ALDH) that metabolizes acetaldehyde to nontoxic acetate. The only way to prevent alcohol intolerance reactions is to avoid alcohol. Alcohol intolerance isn't an allergy. However, in some cases, what seems to be alcohol intolerance may be a reaction to something in an alcoholic beverage, such as chemicals, grains or preservatives. Combining alcohol with certain medications also can cause reactions. In rare instances, an unpleasant reaction to alcohol can be a sign of a serious underlying health problem that requires diagnosis and treatment.