2,5-Dimethyl-2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indol-1-one-d3 is the deuterated version of 2,5-Dimethyl-2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indol-1-one (D480165), which is an impurity of Alosetron (A575500) that is a serotonin 5HT3 receptor antagonist, used in treatment of irritable bowel syndrome.