1) The drug antabuse blocks the action of the enzyme which breaks down acetaldehyde allowing it to build up in the body. This is why drinking alcohol after taking antabuse can make a person extremely ill and can even lead to death.
2) In some people the enzyme which breaks down the acetaldehyde is less efficient than average because it has a slightly different chemical structure. When people who turn bright red and feel ill immediately after drinking alcohol it is because they have this varient of the enzyme. This variant of the enzyme is most common in Asian people--it is estimated to occur in as many as 50% of Japanese people.
The chemical formula for ethyl alcohol is C2H6O. the formula for acetaldehyde is C2H4O. Acetaldehyde is formed when 2 hydrogen atoms are removed from the alcohol molecule. Removing one atom of hydrogen from the acetaldehyde molecule turns it into a harmless acetyl radical which can then be broken down into carbon dioxide and water.
Acetaldehyde is a close relative of formaldehyde which has the chemical formula CH2O. Formaldehyde is formed in the body when it breaks down wood alcohol. This is why wood alcohol is highly poisonous.
Braun, S. (1997). Buzz: The Science and Lore of Alcohol and Caffeine New York: Penguin books.
Thomasson HR, Crabb DW, Edenberg HJ, Li TK. (1993). Alcohol and aldehyde dehydrogenase polymorphisms and alcoholism. Behav Genet. 23(2), 131-6.
Wall TL, Peterson CM, Peterson KP, Johnson ML, Thomasson HR, Cole M, Ehlers CL. (1997). Alcohol metabolism in Asian-American men with genetic polymorphisms of aldehyde dehydrogenase. Ann Intern Med. 127(5), 376-9.