Antihistamine (aka histamine antagonist) inhibits the action of histamine, used to relieve allergies.

• Histamine H1-receptor antagonist, used to Tx allergic reactions in the nose (e.g. itching, runny nose, and sneezing), which are used off-label for insomnia. They are also used to Tx motion sickness or vertigo caused by problems w/ the inner ear, by binding to H1 receptors in mast cells, smooth muscle, and endothelium in the body and tuberomamillary nucleus in the brain. Examples include:
  • 3rd generation antihistamine, as it is less able to pass the BBB and cause sedation:
    • Fexofenadine (Allegra, Telfast)
  • 2nd generation antihistamines, as they are less able to cross the BBB and thus have diminished effects on the CNS, thus less likely to induce drowsiness or interfere w/ memory formation:
    • Cetirizine (Zyrtec)
    • Loratadine (Claritin)
  • 1st generation antihistamines:
    • Quetiapine (an antipsychotic, Seroquel)
    • Doxylamine (Unisom)
    • Diphenhydramine (Benadryl)
• Histamine H2-receptor antagonist, used to Tx gastric acid conditions (e.g. peptic ulcers and acid reflux), by binding to H2 receptors in the upper GI tract, primarily in the stomach

• H2 blocker