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You can't handle your alcohol—at least, not as well as your brother.
And this isn't just because he's bigger than you. Women produce less of the stomach enzyme that breaks down ethanol. After drinking the same number of pints, women have a higher blood alcohol level than men—this is true even when controlling for size differences. The female body also has less water to dilute alcohol, leading to a stronger buzz that night...and a dryer mouth and more intense headache the next morning.