QS Study

Histamine H-receptor Blocker

The agents or drugs which competitively block H-receptors are conventionally called the H-receptor blocker. They are a class of G protein-coupled receptors which bind to histamine as their primary endogenous ligand.


  • H1 = receptor blocker (Antihistamines) – Histamine, Loratidine, Cetrizine.
  • H2 = receptor blacker; – Ranitidine, Famotidine, Nezatidine etc.
  • H3 = receptor blockers – Tioperamide, Impromidine.

H1 – receptor blocker (antihistamines):

H1 receptor antagonists are typically utilized to suppress the body’s histamine-mediated effects in anaphylactoid or anaphylactic reactions.

Mechanism of action –

  1. Antihistamine (Loratidine)
  2. Blocking of the H1 – receptor
  3. Antagonizes all actions of histamine except those mediated by H2


Allergy, Motion sickness, Vomiting, Parkinsonism, Sedation etc.

Fig: H-receptor Blocker

H2 -receptor blocker

H2 antagonists are competitive antagonists at the parietal cell H2 receptor and are typically used to suppress gastric acid secretion.

Mechanism of action

  • Ranitidine (H2 -receptor blocker)
  • Competitively block the H2-receptor
  • ↓ cAMP formation
  • ↓ Secretion of Acid
  • Healing of peptic ulcer.


  • Peptic ulcer disease (PUP) (Duodenal ulcer, Benign gastric ulcer)
  • Reflex esophagus
  • Zollinger sllison’s syndrome.


H3 receptors are expressed in the central nervous system and to a lesser extent the peripheral nervous system, where they act as autoreceptors in presynaptic histaminergic neurons, and also control histamine turnover by feedback inhibition of histamine synthesis and release.