NICOTINE POTENTIATOR IN ADDITIVES. ” Acetaldehyde, which has been studied extensively as a metabolite of ethanol is also present in tobacco smoke in a concentration half that of nicotine. We have previously shown a synergistic interaction of nicotine and acetaldehyde in self-administration in juveniles, which was not observed in adults.”
http://www.nature.com/npp/journal/v32/n9/full/1301327a.html