Chlorpheniramine
Chlorpheniramine maleate
Also known as: Chlor-Trimetron · Chlo-Amine · Aller-Chlor · Allergy Relief · Chlor-Trimeton Allergy · Efidac 24 · QDALL AR · ED-CHLOR-TAN · Pediox-S · Piriton · chlorphenamini maleas · chlorpheniramine maleate
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Adjunctive treatment of chemotherapy of mast cell tumors | 4-8 mg (maximum of 0.5 mg/kg) PO q8-12h PO | PO | q8-12h | — | 🌎 NA |
| General antihistamine use | 4-12 mg (total dose) two to three times daily | PO | BID-TID | — | 🌎 NA |
| General antihistamine use | 2-8 mg (total dose) per dog PO every 12 hours, not to exceed 0.5 mg/kg every 12 hours | PO | q12h | — | 🌎 NA |
| Trial for pruritus in atopic dogs | 0.4-0.8 mg/kg two to three times daily | PO | BID-TID | — | 🌎 NA |
| Pruritus | 0.2-0.8 mg/kg PO 2-3 times a day | PO | BID-TID | — | 🌎 NA |
| Mild sedative | 0.22 mg/kg PO q8h; 4-20 mg (total dose per day) divided q8-12h | PO | q8-12h | — | 🌎 NA |
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| General antihistamine use | 2 mg (total dose) per cat PO every 12 hours | PO | q12h | — | 🌎 NA |
| General antihistamine use | 2-4 mg per cat q12-24h PO | PO | q12-24h | — | 🌎 NA |
| General antihistamine use | 2 mg per cat two to three times daily | PO | BID-TID | — | 🌎 NA |
| Pruritus | 2-4 mg/cat twice daily; rarely may be maintained on once daily dosing. | PO | BID | — | 🌎 NA |
| Pruritus | 0.5-2 mg (total dose; ¼ - ½ of a 4 mg tablet) PO 2-3 times per day | PO | BID-TID | — | 🌎 NA |
| Mild sedative | 1-2 mg per cat q12-24h (low dose), 2-4 mg per cat PO q12-24h (high dose) | PO | q12-24h | — | 🌎 NA |
- General antihistamine use: Most common dosage in cats
- Pruritus: 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.
Ferrets
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| General antihistamine use | 1-2 mg/kg PO 2-3 times a day | PO | BID-TID | — | 🌎 NA |
Birds
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| General use / Feather damaging behavior | One 4 mg tablet in one cup (240 mL; 8 oz.) of bottled water to be used as drinking water; changed daily. | PO | q24h | — | 🌎 NA |
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
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 of action
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) → 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.
Safety & warnings
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
Precautions
Warnings and Precautions
- Anticholinergic Effects: Use with caution in patients with angle closure glaucoma, prostatic hypertrophy, pyloroduodenal or bladder neck obstruction, and COPD (if mucosal secretions are a problem).
- Systemic Disease: Use cautiously in patients with hyperthyroidism, cardiovascular disease, or hypertension.
- Working Dogs: Sedative effects may adversely affect the performance of working dogs.
- Laboratory Testing: Antihistamines can decrease the wheal and flare response to antigen skin testing. It is generally recommended to discontinue antihistamines at least 4 days prior to intradermal allergy testing.
- Nursing/Pregnancy: FDA Category B in humans. It is unknown if it is excreted in milk; use with caution in nursing dams.
Drug interactions
Antihistamines may partially counteract the anticoagulation effects of heparin or warfarin.
May prolong and exacerbate the anticholinergic effects of the antihistamine.
Increased sedation and CNS depression can occur when used concurrently.
Additive CNS depression and sedation
Additive anticholinergic effects (dry mouth, tachycardia, urinary retention)
Monitoring
- Clinical efficacy (reduction in pruritus, allergic signs, or adequate sedation)
- Adverse effects (excessive sedation, GI upset, anticholinergic signs)
Pharmacokinetics
Absorption
In humans, well absorbed after oral administration, but undergoes a relatively high degree of first-pass metabolism in the GI mucosa and liver, resulting in only 25-60% systemic bioavailability. Pharmacokinetics have not been described in domestic species.
Distribution
Well distributed after IV injection; highest distribution occurs in the lungs, heart, kidneys, brain, small intestine, and spleen. In humans, the apparent steady-state volume of distribution is 2.5-3.2 L/kg and about 70% is bound to plasma proteins.
Metabolism
Metabolized in the liver.
Elimination
Practically all the drug (as metabolites and unchanged drug) is excreted in the urine.
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.
Available products
Formulations
- Oral tablets
- Chewable tablets
- Extended-release tablets
- Extended-release capsules
- Oral syrup
- Oral suspension
Human-labeled
- Chlorpheniramine Maleate Oral Tablets: 2 mg (chewable), 4 mg
- Chlorpheniramine Maleate Extended-release Tablets & Capsules: 8 mg, 12 mg & 16 mg
- Chlorpheniramine Caplets: 8 mg (as tannate)
- Chlorpheniramine Maleate Oral Syrup: 2 mg/5 mL
- Chlorpheniramine Maleate Oral Suspension: 4 mg/5 mL
Regulatory status
Human formulations used under the cascade.
Classified as POM (Prescription Only Medicine) for injectables and GSL (General Sales List) for certain oral forms.
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Store tablets and sustained-release tablets in tight containers. Sustained-release capsules should be stored in well-closed containers. Oral solution should be stored in light-resistant containers; avoid freezing. All chlorpheniramine products should be stored at room temperature (15-30°C).
Client information
Information for Pet Owners
- Expected Effects: This medication is commonly used to treat allergies and itching. It may cause your pet to become drowsy or sleepy.
- Cats: While dogs usually get sleepy, cats may occasionally experience the opposite effect and become hyperactive or excited.
- Administration: If you are using a sustained-release (long-acting) product, do not crush the pill or allow your pet to chew it. The contents of sustained-release capsules may be sprinkled on food, but must be eaten immediately before they dissolve.
- Human Products: Chlorpheniramine is an FDA-approved human medication often found over-the-counter. Only use the exact product and dose recommended by your veterinarian, as many human cold/allergy products contain additional ingredients (like decongestants or pain relievers) that can be highly toxic to pets.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.