Mechanism of action of cimetidine: 1. immune enhancing effect. 2. increase the number of suppressive T cells depleting the number of helper T cells. This leads to a decrease in IGE and thus reduces allergic symptoms. 3. Inhibition of mast cell secretory mediators. 4. Anti-oxidant effect. 5. Anti-androgen effect. 6. Anti-tumor effect. Clinical applications: (1) Chronic urticaria (2) Acute allergic urticaria with or without hypotension. (3) pigmented urticaria (4) female androgenic alopecia (5) herpes zoster (6) other solar urticaria oral and lip herpes simplex, etc.