Cetirizine is an over-the-counter antihistamine often used for allergies, and it has also been tried in people with Long COVID who may have mast cell activation syndrome (MCAS)–like symptoms. These can include flushing, itching, rashes, swelling, stomach upset, or even brain fog triggered by immune system overactivity. Some studies and patient reports suggest antihistamines like cetirizine may help reduce fatigue and cognitive problems linked to MCAS in Long COVID. It is generally safe and well tolerated, though it can cause drowsiness or dry mouth in some people. While not a cure, cetirizine may offer symptom relief for patients with Long COVID who show signs of mast cell involvement.