To explore whether niacin protects against inflammatory bowel diseases (IBDs) through releasing PGD2, we first examined niacin‐induced PGD2 production in colon tissues and urinary secretion of PGD2 metabolites‐ 11,15‐Dioxo‐9α‐hydroxy‐2,3,4,5‐tetranorprostan‐1,20‐dioic acid (tetranor PGDM) from DSS‐induced colitis mouse model by using mass spectrometry analysis. Here, HPGDS is linked to colitis.