What would be the effect of someone drinking if they were allergic to alcohol?

Allergic? Maybe not.: They probably aren't technically "allergic". Some people have exaggerated nasal congestion, or flushing. Drinking while taking certain prescription drugs can trigger rapid heart rate, flushing and nausea, that might look similar to anaphylaxis, but is not (I was called to First class once for this to attend to a smiling but red-faced senior who frightened the flight attendants)