Chlorpheniramine
Chlorpheniramine is a first-generation alkylamine antihistamine used primarily in veterinary medicine for its antihistamine and antipruritic (anti-itch) effects. **Key Clinical Points:** * **First-Generation Antihistamine:** Because it crosses the blood-brain barrier, it frequently causes CNS depression (sedation), which is occasionally utilized therapeutically as a mild sedative. * **Feline Pruritus:** It is one of the more commonly used antihistamines in cats for managing allergic skin disease and pruritus. * **Mast Cell Tumors:** Often used as an adjunctive treatment in the chemotherapy protocols for mast cell tumors to block the effects of systemic histamine release. * **Paradoxical Excitement:** While it typically causes sedation in dogs, cats may exhibit paradoxical excitement.
Mechanism: Chlorpheniramine acts as a competitive **H1-receptor antagonist**. * **Histamine Blockade:** It competitively inhibits histamine from binding to **H1 receptors** on effector cells (such as smooth muscle and endothelial cells) โ prevents histamine-induced vasodilation, increased capillary permeability, and smooth muscle spasms. * **Note:** It does *not* inactivate histamine or prevent its release from mast cells; it only blocks its action at the receptor site. * **Additional Properties:** Like many first-generation antihistamines, it possesses varying degrees of **anticholinergic** (muscarinic blockade) and **CNS depressant** activity.
Dosing by species
- General antihistamine use ยท 2 mg (total dose) per cat PO every 12 hours ยท PO ยท q12h
- General antihistamine use ยท 2-4 mg per cat q12-24h PO ยท PO ยท q12-24h
- General antihistamine use ยท 2 mg per cat two to three times daily ยท PO ยท BID-TID ยท Most common dosage in cats
- Pruritus ยท 2-4 mg/cat twice daily; rarely may be maintained on once daily dosing. ยท PO ยท BID ยท Palatability may be enhanced by dipping the split tablet into tuna fish 'juice', butter or petrolatum; placing split tablets into empty gelatin capsules or sprinkling or mixing timed release beads (partial contents of an 8 mg capsule) with food.
- Pruritus ยท 0.5-2 mg (total dose; ยผ - ยฝ of a 4 mg tablet) PO 2-3 times per day ยท PO ยท BID-TID
- Mild sedative ยท 1-2 mg per cat q12-24h (low dose), 2-4 mg per cat PO q12-24h (high dose) ยท PO ยท q12-24h
- General antihistamine use ยท 1-2 mg/kg PO 2-3 times a day ยท PO ยท BID-TID
- General use / Feather damaging behavior ยท One 4 mg tablet in one cup (240 mL; 8 oz.) of bottled water to be used as drinking water; changed daily. ยท PO ยท q24h
Routes of administration
Contraindications
- Hypersensitivity to chlorpheniramine or other alkylamine antihistamines
Adverse effects
- CNS depression (lethargy, somnolence, sedation)
- GI effects (diarrhea, vomiting, anorexia)
- Anticholinergic effects (dry mouth, urinary retention)
- Paradoxical excitement (especially in cats)
- Decreased performance in working dogs due to sedation
Drug interactions
- Anticoagulants (heparin, warfarin) ยท Antihistamines may partially counteract the anticoagulation effects of heparin or warfarin.
- MAO Inhibitors (amitraz, selegiline) ยท May prolong and exacerbate the anticholinergic effects of the antihistamine.
- CNS Depressants ยท Increased sedation and CNS depression can occur when used concurrently.
- CNS Depressants (e.g., opioids, sedatives, barbiturates) ยท Additive CNS depression and sedation ยท moderate
- Anticholinergic drugs ยท Additive anticholinergic effects (dry mouth, tachycardia, urinary retention) ยท moderate
Monitoring
- Clinical efficacy (reduction in pruritus, allergic signs, or adequate sedation)
- Adverse effects (excessive sedation, GI upset, anticholinergic signs)
Overdose
### Overdose Signs Overdosage may cause **CNS stimulation** (ranging from excitement to seizures) or **CNS depression** (lethargy to coma), anticholinergic effects, respiratory depression, and death. > *Case report:* A 9-month-old dachshund ingesting 25 mg/kg showed signs of ataxia, tremors, bradycardia, coma, and cardiac arrest, dying within 11 hours of ingestion. ### Treatment * **Decontamination:** Empty the gut using standard protocols if ingestion was oral. Induce emesis if the patient is alert and CNS status is stable. Follow with a saline cathartic and/or activated charcoal. * **Supportive Care:** Treat other clinical signs symptomatically. * **Seizure Control:** **Phenytoin (IV)** is recommended in the treatment of seizures caused by antihistamine overdoses in humans; **barbiturates and diazepam should be avoided**.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.