Clemastine
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: 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.
Dosing by species
- Antihistamine · 0.68 mg per cat PO twice daily · PO · q12h
- Antihistamine · 0.34-0.68 mg per cat PO q12h · PO · q12h
- Atopy · 0.15 mg/kg PO q 12 hrs · PO · q12h · Efficacy may be increased by combining with omega 3 fatty acids.
- Allergic conditions (Historical dosing - Efficacy doubtful) · 0.05-0.1 mg/kg PO q12h · PO · q12h · 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.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
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)
Drug interactions
- CNS Depressant Medications (e.g., barbiturates, tranquilizers) · Additive CNS depression may occur.
- Monoamine Oxidase Inhibitors (e.g., furazolidone, amitraz, selegiline) · May intensify the anticholinergic effects of clemastine.
- CNS Depressants (e.g., phenobarbital, gabapentin, opioids) · Additive CNS depression and sedation · moderate
- Anticholinergic drugs · Additive anticholinergic effects (dry mouth, urinary retention) · moderate
Monitoring
- Clinical efficacy (reduction in pruritus, allergic signs)
- Adverse effects (sedation, hyperactivity, anticholinergic signs)
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.
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.