When the normal alcohol metabolic pathway (alcohol dehydrogenase/aldehyde dehydrogenase) is saturated (remember alcohol, aspirin, and phenytoin metabolism follow zero order kinetics which means they are saturable), the cytochrome P450 pathway starts metabolizing alcohol. For a person not used to drinking alcohol (acute alcohol intake), the low capacity cytochrome P450 is easy to saturate which means it is no longer available for metabolizing other toxins and drugs (aka it's metabolic and detoxification actions are inhibited). Because CYp450 is an inducible system, for someone who is a chronic drinker, CYP450 is induced so that the capacity of the system increases due to increased enzyme production in order to both handle the chronic alcohol intake and to perform the other essential metabolic/detox functions of the CYP450 system.
As an aside, the same CYP450 enzyme (CYP2E1) that metabolizes alcohol is also responsible for metabolizing acetaminophen.
As another aside, I know you said you were studying for the USMLE as you go along in MS2 but you should be cautious not to use your time inefficiently. If this stuff is new to you, then don't try to pre-study. Go at the pace of your classes and follow the materials as it's taught. Learning stuff piecemeal through pre-studying review materials is not only inefficient but it really doesn't help with making the essential connections that step1 wants you to know.
