Alcohol, like DXM, also effects the NMDA receptors, which are responsible for most of DXM's subjective effects(this is also the reason why people black out after drinking too much). How much it will effect you is individual, and strongly advise you to take it easy - remember you can always take another dose, but you can't take a dose back. Also, if the DXM you are taking has acetaminophen with it, do not consume alcohol concurently, as you run the very real risk of acute liver failure.