General physiology

Target: histamine H1 receptors

Location of H1 receptors
- Smooth muscles (especially bronchial and nasopharyngeal lining)
- Vascular endothelial cell surfaces
- Central nervous system
Histamine effects on H1 receptors
- ↑ Capillary dilation and permeability → hypotension and edema
- ↑ Bronchiolar smooth muscle contraction (via IP3 and DAG release) → bronchoconstriction
- ↑ Nasal and bronchial mucus production

Drug
- Diphenhydramine
- Meclizine
- Doxylamine
- Promethazine
- Clemastine
- Dimenhydrinate
- Chlorpheniramine
- Dimetindene
Uses
- Antiallergic agent
  - Allergic rhinitis and conjunctivitis
  - Contact dermatitis
  - Hay fever, urticaria, angioedema, and rhinorrhea
- Antiemetic agent
- Sedative agent
- Anaphylactic shock
- Motion sickness
Characteristics
- Strong sedative action
- Anticholinergic effects, e.g., dry mouth and eyes, mydriasis, urinary retention, tachycardia, dizziness, tinnitus (mostly with first-generation antihistamines)
- Anti-α-adrenergic effects, e.g, postural hypotension (can lead to falls), weight gain

Drug
- Loratadine
- Desloratadine
- Cetirizine
- Fexofenadine
  - Fox, satyr, and rat: 2nd generation H1 blockers, Fexofenadine, cetirizine, loratadine
Uses
- Antiallergic agent
Characteristics
- Nonsedative/mildly sedative
  - These antihistamines usually do not cross the blood-brain barrier and act mainly outside the central nervous system.

Drug
- Cimetidine
- Famotidine
- Nizatidine
Uses
- To reduce production of stomach acid
Characteristics
- Usually used as second-line treatment or in combination with proton pump inhibitors (PPIs)