Histamine intolerance is a widespread phenomenon that can mimic allergy. It plays a role in many diverse conditions including IBS and inflammatory bowel diseases, asthma, postural orthostatic tachycardia syndrome (POTS), complications of pregnancy, drug hypersensitivity, and intestinal permeability (by means of which it contributes to autoimmunity). It can be objectively evaluated with serum DAO activity. HI can be ameliorated by supplementation with DAO. Practitioners need to be astute in its diagnosis and management.