Cyproheptadine
H1 antihistamines
H1 antihistamines
A sedating antihistamine that also blocks serotonin receptors; known for increasing appetite and as an antidote in serotonin syndrome.
Quick facts
- Group
- H1 antihistamines
- Target
- Histamine H1 receptor
- Used for
- Allergic rhinitis · Itchy allergic skin conditions
- Key effects
- Drowsiness · Increased appetite and weight gain
- Monitor
- Sedation · Weight · In serotonin syndrome
- ATC
- R06AX02
No doses: the clinician sets them by the current guideline.
Mechanism of action
The figure shows doxepin, a tricyclic first-generation H1 blocker, in the human H1 receptor; cyproheptadine, also tricyclic, binds the same site.
Cyproheptadine is a first-generation antihistamine that competes with both histamine (H1) and serotonin (mainly 5-HT2) for their receptors, with anticholinergic and sedative effects. Serotonin blockade is why it is used to treat serotonin syndrome and why it can increase appetite and weight.
- Cyproheptadine blocks H1 receptors: less allergy, sedation
- It also blocks 5-HT2 serotonin receptors
- Serotonin blockade counters serotonin excess and raises appetite
- Muscarinic block adds dry mouth and urinary retention
Indications
- Allergic rhinitis and allergic conjunctivitis Allergic rhinitis →
- Itchy allergic skin conditions, hives and cold urticaria Urticaria and angioedema →
- Add-on after epinephrine in anaphylaxis Anaphylaxis →
- Serotonin syndrome (off-label antidote) Serotonin syndrome →
Adverse effects
Common
- Drowsiness
- Increased appetite and weight gain
- Dry mouth, constipation, blurred vision
- Dizziness
Contraindications
- Newborns and premature infants; breastfeeding
- Therapy with MAO inhibitors
- Angle-closure glaucoma, stenosing peptic ulcer, pyloroduodenal or bladder neck obstruction, symptomatic prostate enlargement
- Elderly, debilitated patients
Key interactions
- MAO inhibitors
- Prolong and intensify the anticholinergic effects
- Alcohol, benzodiazepines, opioids and other CNS depressants
- Additive sedation and slowed breathing
What is monitored
- Sedation
- Weight
- In serotonin syndrome: temperature, heart rate, rigidity and clonus
Administration and safety
- Oral tablets or syrup; in serotonin syndrome the tablets may be crushed and given through a nasogastric tube
Pregnancy and breastfeeding
Human data are limited; it is contraindicated in breastfeeding and in newborns, and use in pregnancy is weighed with the clinician.
Nursing care
- Assess sedation and fall risk
- Record weight when given for a long time
- Keep away from children; overdose is dangerous in the young
- Teach not to drive or operate machinery until the effect is known, and to avoid alcohol
Pharmacokinetics
- Absorption
- Well absorbed by mouth
- Metabolism
- Extensive liver metabolism (glucuronidation)
- Elimination
- Urine, as metabolites
Test yourself
Think of the answer, then tap the question to see it.
Besides H1, which receptor does cyproheptadine block that makes it useful in serotonin syndrome?
Serotonin 5-HT2 receptors.
Which typical side effect of cyproheptadine is sometimes used on purpose?
Increased appetite and weight gain.
Why is cyproheptadine contraindicated with MAO inhibitors?
MAO inhibitors prolong and intensify its anticholinergic effects.
Connections
← Back to the class: H1 antihistamines
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - CYPROHEPTADINE HYDROCHLORIDE tablet (DailyMed (U.S. National Library of Medicine), 2026); Cyproheptadine: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026).
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Representative model, for educational purposes only.