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Alcohol dilates peripheral blood vessels, but has been known to actually. constrict some blood vessels in the brain, lowering the oxygen content. Alcohol dilates peripheral blood vessels, but has been known to actually  constrict some blood vessels in the brain, lowering the oxygen content.
When an alcohol metabolite called acetaldehyde builds up in your system, it causes your blood vessels to constrict, which weakens blood flow and makes you look like a guy whoâs thrown one too many plates on his barbell, explains study coauthor Jong-Sung Kim, M.D., Ph.D., of Chungnam National University.