Histamine receptor 1 inhibitors

Histamine receptor 2 inhibitors

Mechanism of action:
Mechanism of action:

Histamine has 2 types of receptors. One is Histamine 1 and other is histamine 2. Activation of histamine 1 receptors produce classic symptoms of inflammation and activation of histamine 2 increase the secretion of HCL by parietal cells in the stomach.

Histamine 1 is a chemical mediator of inflammation. When histamine released from the mast cells and basophills, histamine reaches its receptors to cause itching and nasal congestion. In severe reaction, histamine release can cause the bronchoconstruction, edema, hypotension, and other symptoms of the anaphylaxis.

Histamine-1 receptors blocks 1 receptors and that’s how they reduce the inflammation and allergy reaction. H1 receptor antagonists and antihistamine terms are used interchangeably.

These drugs compete with histamine for histamine receptor sites. By occupying the histamine receptor sites, they prevent histamine from causing allergic symptoms. Antihistamines are most effective when taken continuously during the allergy season.

In addition to producing their antihistamine effects, these drugs also cause typical anticholinergics effects.


  • Allergic rhinitis
  • Vertigo and motion sickness (ex: meclizine, and dimenhydrinate)

Nausea resulting from vertigo or motion sickness responds well to antihistamines. These drugs act by suppressing the vomiting center in the medulla and depressing neurons of the vestibular apparatus of the inner ears.

  • Parkinson’s disease: (Due to anticholinergics properties ex: diphenhydramine)
  • Tremors
  • Insomnia (For the initial treatment then after patient usually become tolerance ex: diphenhydramine and doxylamine)
  • Urticartia and other skin rashes

Urticartia or hives is often caused by the release of histamine, thus the condition responds well to H1-receptors antagonists.

First generation agents (mostly produce sedation)

  • Azelastine
  • Brompheniramine
  • Chlorpheniramine
  • Clemastine
  • Cyproheptadine
  • Dexbrompheniramine
  • Dexchlorpheniramine
  • Dimenhydrinate
  • Diphenhydramine
  • Promethazine
  • Triprolidine

Second-generation agents (Mostly they are non-sedating)

  • Cetirizine
  • Desloratadine
  • Fexofenadine
  • Levocetirizine
  • Loratadine
  • Olopatadine

Side effects:

  • Dry mouth
  • Headache
  • Dizziness
  • Urinary retention
  • Thickening of bronchial secretion
  • Nausea
  • Vomiting
  • Paradoxical excitation
  • Sedation
  • Hypersensitivity
  • Hypotension
  • Extrapyramidal symptoms
  • Agranulocytosis
  • Respiratory depression


  • Should not be taken with alcohol or opioids causes increased sedation
  • Avoid taking other drugs that cause drowsiness, such as alcohol, sleep preparations, sedatives, or tranquilizers.
  • Avoid taking MAOIs (for example, isocarboxazid [Marplan], phenelzine sulfate [Nardil], or tranylcypromine [Parnate]) within 14 days of antihistamines.
  • Do not take these drugs with anticholinergics drugs and people with narrow angle glaucoma, urinary retention, prostate hypertrophy and bowel obstruction.