Foods are consumed and then broken down by enzymes in our bodies. Different foods are broken down (digested) in different parts of the body, such as in the intestines or in the stomach. Alcohol, which is easily digestible, is absorbed directly into the blood stream via the gastrointestinal tract. Alcohol is a toxic substance and must be broken down and oxidized by aldehyde dehydrogenase (ALDH) enzymes in the liver. This process turns the alcohol into acetic acid (vinegar), from which some energy can be obtained.

However, some people have defunct ALDH genes. As a result, they produce inactive ALDH enzymes that don't break down the alcohol into acetic vinegar. People with this gene deficiency suffer two-fold from allergic symptoms to alcohol. First, the body produces histamines in response to the presence of the alcohol that the body is unable to digest. Secondly, ALDH is the enzyme that is supposed to metabolise histamine. As a consequence, people with alcohol allergies will suffer from excessive amounts of histamine in their systems, leading to allergy-like symptoms, such as nasal congestion and mild flushing immediately upon consuming even minute amounts of alcohol.

Although allergies to alcohol are rare, some people will respond to the consumption of alcohol with one or more allergy symptom. While these symptoms will range in severity among different people, nasal allergies such as rhinitis are increasingly linked to alcohol intolerance due to the inflammatory effect of the histamine produced in response to the presence of alcohol. The histamine causes the blood cells in the nasal region to dilate, resulting in mucous, nasal congestion, sneezing and a runny nose.

Research has also shown that women who drink regularly (14 or more alcoholic beverages per week) are at increased risk of developing perennial nasal allergies and exacerbating existing allergies.