The answer to your question is 'Tolerance'

Question

My brother is a functional alcoholic. He drinks every night and gets up to go to work 5 days a week. He is visiting right now and last night we shared a case of beers. This is not something I do very often but it is something he does every day. Today I had the worst hangover I can remember ever having in my life. It is 8 in the evening now and I am barely able to take in solid foods even now! My brother has already been drinking for hours and he was up at 8 this morning working on the broken lawnmower I had been complaining about – right as rain. Why can he drink and not get hungover? I am trying to understand why he drinks so much. Do alcoholics become so used to alcohol that they no longer become hung over after drinking. I cannot imagine drinking again for a long while after the way I felt today.

Scott Graham · Accepted Answer

Specifically, it is called, consumption-induced tolerance which includes both speed of recovery when drinking and needing higher quantities of alcohol to get the same effects.

You also hinted at a second question: "I am trying to understand why he drinks so much."

You could easily drive yourself nuts trying to answer that question. IMHO, the answer doesn't matter much at all (I think this is true for most "why" questions).

You might get more by asking "what" -- "What can I do to help him?" "What can he do to help himself?" etc.

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