5-hydroxy Tryptophol is a metabolite of tryptophan that is formed by the alcohol dehydrogenase-catalyzed reduction of the serotonin intermediate, 5-hydroxyindoleacetaldehyde. Depending on the tissue NAD/NADH ratio, the acetaldehyde intermediate can also be oxidized by aldehyde dehydrogenase to form 5-hydroxyindoleacetic acid. Thus, a ratio of 5-hydroxy tryptophol to 5-hydroxyindoleacetic acid has been used as a biomarker for recent alcohol consumption.