Is it possible that an alcoholic has an allergy to alcohol that causes a chemical reaction and combined with mental obsession means that they cannot drink safely?

The term "allergy" is an old one and is no longer commonly used. There do seem to be differences in how people metabolize and react to alcohol that are at least mostly genetic. An alcoholic is one who is addicted to the liquid drug alcohol. There are many ways to look at it, but this is fundamental.