Among the most popular over-the-counter (OTC) allergy meds are Zyrtec and Claritin.
Unlike the allergy medications Claritin, Zyrtec or Allegra, this medication blocks histamine receptors in the brain.
Histamine is necessary to promote wakefulness, motivation and goal-directed behaviors; when the receptors for histamine are blocked, drowsiness occurs.
Many companies have marketed diphenhydramine and doxylamine (another sedating antihistamine) for insomnia under different brand names.
Once the mast cell-antibody-antigen complex is formed, a complex series of events occurs that eventually leads to cell degranulation and the release of histamine (and other chemical mediators) from the mast cell or basophil.
Once released, the histamine can react with local or widespread tissues through histamine receptors.