Histamine intolerance and Mast Cell Activation Syndrome (MCAS) result when the body cannot properly break down histamine or when mast cells release histamine inappropriately. This can lead to a wide range of symptoms—from skin flushing and itching to brain fog, headaches, and IBS-like symptoms. Functional medicine recognizes that underlying issues like gut dysbiosis, mold exposure, stress, estrogen dominance, or nutrient deficiencies often contribute to histamine overload or mast cell instability. Treatment focuses on reducing histamine load, supporting DAO enzyme function, healing the gut, and calming the nervous and immune systems.