Cyproheptadin mechanism of action
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cyproheptadine mechanism of action H1 serotonin antagonist pharmacology
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Here is a detailed breakdown of cyproheptadine's mechanism of action, drawn from Katzung's Basic and Clinical Pharmacology (16th ed.) and Kaplan & Sadock's Synopsis of Psychiatry:
Cyproheptadine - Mechanism of Action
Chemical Structure
Cyproheptadine resembles phenothiazine antihistaminic agents in chemical structure, which explains its broad receptor-blocking profile.
Receptor Targets
| Receptor | Action | Clinical Consequence |
|---|---|---|
| H1 histamine receptor | Competitive antagonist | Antiallergic, antipruritic, relieves urticaria |
| 5-HT2 (serotonin) receptor | Potent antagonist (especially 5-HT2A/2C) | Appetite stimulation, treatment of serotonin syndrome, reduced muscle spasms |
| Muscarinic (cholinergic) | Antagonist (anticholinergic) | Sedation, dry mouth, blurred vision, urinary retention |
| Calcium channels | Mild blocking activity | Contributes to smooth muscle relaxation |
Key Mechanistic Points
-
H1 blockade: Prevents histamine-mediated smooth muscle effects (bronchoconstriction, vasodilation, pruritus). Notably, it does not block histamine-stimulated gastric acid secretion (that requires H2 antagonism).
-
5-HT2 blockade: Blocks serotonin's smooth muscle effects. This is the basis for its use in:
- Carcinoid syndrome - counters serotonin-driven flushing and diarrhea
- Serotonin syndrome - mild-to-moderate cases (not severe/critically ill patients)
- Appetite stimulation - 5-HT2C blockade in the hypothalamus disinhibits appetite centers
- Muscle spasm after spinal cord injury - constitutive 5-HT2C activity is blocked, reducing abnormal Ca²⁺ currents
-
5-HT2A blockade specifically: Accounts for its off-label use as a "trip killer" against serotonergic psychedelics, and improvement in SSRI-induced anorgasmia (4-16 mg, 1-2 hours before sexual activity).
-
CNS penetration: Crosses the blood-brain barrier (first-generation drug), causing sedation via central H1 blockade.
Clinical Uses Derived from Mechanism
- Allergic conditions (urticaria, rhinitis, cold-induced urticaria) - H1 antagonism
- Appetite stimulation (anorexia nervosa, cancer cachexia, wasting in children) - 5-HT2C hypothalamic blockade
- Serotonin syndrome (mild-moderate) - 5-HT2A/2C antagonism
- Carcinoid tumor (smooth muscle manifestations) - dual H1 + 5-HT2 antagonism
- PTSD nightmares - antiserotonergic effect
- Akathisia (neuroleptic-induced) - serotonin antagonism
- SSRI-induced anorgasmia - 5-HT2A blockade
Standard Dosing
Adult dose: 12-16 mg/day orally in 3-4 divided doses (4 mg per dose). Available only in tablet form; in unconscious patients, tablets must be crushed and given via nasogastric tube.
- Katzung's Basic and Clinical Pharmacology, 16th Ed., p. 458 | Kaplan & Sadock's Synopsis of Psychiatry
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