Clemastine
Clemastine fumarate
Also known as: Tavist Allergy · Dayhist-1 · Agasten · Aller-Eze · Antihist-1 · Clamist · Contac 12 Hour Allergy · Tavegil · Tavegyl · meclastine fumarate · mecloprodin fumarate · clemastini fumaras · HS-592 · Clemastine fumarate
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 |
|---|---|---|---|---|---|
| Allergic conditions (Historical dosing - Efficacy doubtful) | 0.05-0.1 mg/kg PO q12h | PO | q12h | — | 🌎 NA |
- Allergic conditions (Historical dosing - Efficacy doubtful): Oral bioavailability is <5%. An oral dose of 0.5 mg/kg only slightly inhibited wheal formation. Further dosing studies are needed before recommending therapeutic oral use.
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Antihistamine | 0.68 mg per cat PO twice daily | PO | q12h | — | 🌎 NA |
| Antihistamine | 0.34-0.68 mg per cat PO q12h | PO | q12h | — | 🌎 NA |
| Atopy | 0.15 mg/kg PO q 12 hrs | PO | q12h | — | 🌎 NA |
- Atopy: Efficacy may be increased by combining with omega 3 fatty acids.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Clemastine is a first-generation ethanolamine antihistamine used for the symptomatic relief of histamine-mediated allergic conditions.
Key pharmacological characteristics include:
- Higher anticholinergic activity compared to many other antihistamines.
- Lower sedative profile than average first-generation antihistamines.
- Poor oral bioavailability in dogs (3%) and horses (3-4%), making oral administration highly questionable for achieving therapeutic systemic exposure in these species.
Clinical Pearl: Recent pharmacokinetic studies suggest that traditional oral dosing regimens in dogs and horses are likely subtherapeutic. Intravenous administration may be required for reliable efficacy in these species.
Mechanism of action
Like other first-generation antihistamines, clemastine acts as a competitive antagonist (or inverse agonist) at H1-receptors.
- Mechanism: Competes with histamine for H1-receptor sites on effector cells (smooth muscle, endothelium, CNS) → prevents histamine-induced bronchoconstriction, vasodilation, and increased capillary permeability.
- Note: It does not block the release of histamine from mast cells or basophils, but rather antagonizes its downstream effects.
- Exhibits significant anticholinergic (antimuscarinic) activity, which contributes to both its drying effects on mucous membranes and potential side effects.
Safety & warnings
Contraindications
- Known hypersensitivity to clemastine
Adverse effects
- Sedation (dogs)
- Paradoxical hyperactivity (dogs)
- Anticholinergic effects: dry mucous membranes, tachycardia, urinary retention
- Diarrhea (cats)
- Fixed drug reaction (cats - rare)
Precautions
Use with caution in patients with prostatic hypertrophy, bladder neck obstruction, severe cardiac failure, angle-closure glaucoma, or pyeloduodenal obstruction. Antihistamines should be discontinued 3-7 days before intradermal skin testing to prevent false-negative wheal and flare responses. Use with caution in pregnant or nursing animals, especially with newborns, as the drug enters maternal milk.
Drug interactions
Additive CNS depression may occur.
May intensify the anticholinergic effects of clemastine.
Additive CNS depression and sedation
Additive anticholinergic effects (dry mouth, urinary retention)
Monitoring
- Clinical efficacy (reduction in pruritus, allergic signs)
- Adverse effects (sedation, hyperactivity, anticholinergic signs)
Pharmacokinetics
Half-life
Absorption
Dogs: Very low oral bioavailability (3%). Horses: Poor oral bioavailability (3-4%). Humans: Variable bioavailability (20-70%).
Distribution
Dogs: High volume of distribution (13.4 L/kg); 98% protein bound. Horses: Volume of distribution at steady-state is 3.8 L/kg. Distributes into milk.
Metabolism
Appears to be extensively metabolized (based on human data).
Elimination
Metabolites are eliminated in the urine. Clearance in dogs: 2.1 L/hr/kg. Clearance in horses: 0.79 L/hr/kg.
Overdose
There are no specific antidotes for clemastine overdose.
- Management: Handle significant overdoses using standard gut-emptying protocols (emesis/gastric lavage) when appropriate, followed by supportive therapy.
- Clinical Signs: Overdose signs are extensions of the drug's side effects, principally CNS depression (though CNS stimulation/seizures may occur), severe anticholinergic effects (severe drying of mucous membranes, tachycardia, urinary retention, hyperthermia), and possibly hypotension.
- Specific Treatments: Physostigmine may be considered to treat serious CNS anticholinergic effects. Diazepam can be employed to treat seizures if necessary.
Available products
Formulations
- Oral tablets
- Oral syrup
Human-labeled
- Clemastine Fumarate Oral Tablets: 1.34 mg (equivalent to 1 mg clemastine), 2.68 mg (equivalent to 2 mg clemastine)
- Clemastine Oral Syrup: 0.67 mg/5 mL (equivalent to 0.5 mg clemastine)
Regulatory status
Available as a human product used off-label in veterinary medicine.
Classified as GSL (General Sales List) in the UK, meaning it can be sold without a prescription.
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Oral tablets and solution should be stored in tight, light-resistant containers at room temperature.
Client information
Clemastine is an antihistamine used to help manage allergies.
- Symptom Relief: Understand that antihistamines may be useful for the symptomatic relief of allergic signs, but they are not a cure for the underlying disease.
- Side Effects: May cause mild sleepiness or, conversely, unexpected hyperactivity in some pets. Dry mouth is also possible.
- Cats: Watch for diarrhea, which is the most commonly reported side effect in felines.
- Administration: Give exactly as prescribed. Do not increase the dose without consulting your veterinarian.
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.