Pyrilamine Maleate
**Pyrilamine maleate** (also known as mepyramine) is a first-generation, ethylenediamine-class antihistamine used in veterinary medicine primarily to reduce or prevent histamine-mediated adverse effects. Key clinical points: * **Primary Use**: Predominantly utilized in **equine practice** for the management of allergic conditions such as insect bite hypersensitivity, hives (urticaria), and as an adjunct in respiratory conditions like equine asthma (heaves). * **Advantages**: Compared to other first-generation antihistamines (like diphenhydramine), pyrilamine is considered to be **less sedating** and possesses **fewer anticholinergic side effects**. * **Formulations**: Often found in combination products (e.g., with pseudoephedrine or guaifenesin) as oral granules for horses, or as an injectable solution. * **Regulatory Note**: In equine sports, it is classified as an ARCI Class 3 drug and requires appropriate withdrawal times before competition.
Mechanism: Pyrilamine acts as a **competitive antagonist (inverse agonist)** at **H1-receptors**. * **Mechanism**: Allergic trigger โ Mast cell degranulation โ Histamine release. Pyrilamine competes with free histamine for binding sites on **H1-receptors** located on smooth muscle, endothelium, and in the CNS. * **Effects**: By blocking these receptors, it prevents histamine-induced bronchoconstriction, vasodilation, and increased capillary permeability (which leads to edema/hives). * **Limitations**: It does **not** inhibit the release of histamine from mast cells, nor does it inactivate histamine that is already bound. Therefore, it is more effective as a preventative rather than a rescue treatment during acute anaphylaxis. * **Secondary Actions**: Like other first-generation antihistamines, it can cross the blood-brain barrier, leading to mild CNS depression (sedation) or paradoxical CNS stimulation, and possesses mild anticholinergic properties.
Dosing by species
- Antihistamine ยท 0.25-0.5 gram IM ยท IM ยท Unknown
- Antihistamine ยท 0.5-1.5 grams IM ยท IM ยท Unknown
- Adjunctive treatment of aseptic laminitis ยท 55-110 mg/100 kg IV or IM ยท IV, IM ยท Unknown
- Antihistamine ยท 0.88-1.32 mg/kg (2-3 mL of 20 mg/mL solution per 100 lbs body weight) IV (slowly), IM or SC ยท IV, IM, SC ยท q6-12h ยท May repeat in 6-12 hours if necessary. NOTE: ARCI UCGFS CLASS 3 DRUG
- Antihistamine (Foals) ยท 0.44 mg/kg (1 mL of 20 mg/mL solution per 100 lbs. body weight) IV (slowly), IM or SC ยท IV, IM, SC ยท q6-12h ยท May repeat in 6-12 hours if necessary.
- Antihistamine ยท 1 mg/kg IV , IM or SC ยท IV, IM, SC ยท Unknown
- Antihistamine ยท 0.5-1.5 grams IM ยท IM ยท Unknown
- Antihistamine ยท 0.25-0.5 gram IM ยท IM ยท Unknown
- Antihistamine ยท 12.5-25 mg PO four times a day ยท PO ยท q6h
Routes of administration
Contraindications
- Should not supersede the use of other emergency drugs and procedures (e.g., epinephrine for acute anaphylaxis)
Adverse effects
- CNS stimulation (nervousness, insomnia, convulsions, tremors, ataxia)
- Palpitation
- GI disturbances
- CNS depression (sedation)
- Muscular weakness
- Anorexia
- Lassitude
- Incoordination
Drug interactions
- Anticoagulants (heparin, warfarin) ยท Antihistamines may partially counteract the anticoagulation effects of heparin or warfarin.
- CNS Depressant Drugs ยท Increased sedation can occur if pyrilamine is combined with other CNS depressant drugs.
- Epinephrine ยท Pyrilamine may enhance the effects of epinephrine.
Monitoring
- Clinical efficacy (reduction in allergic signs, hives, or respiratory effort)
- Adverse effects (monitor for signs of CNS stimulation or excessive sedation)
Overdose
Treatment of overdosage is **supportive and symptomatic**. * **Seizure Management**: Phenytoin (IV) is recommended in the treatment of seizures caused by antihistamine overdose in humans. Most toxicologists recommend **avoiding** barbiturates and diazepam, though one veterinary manufacturer historically suggested 'careful titration' of barbiturates. * **CNS Depression**: Avoid the use of CNS stimulants (analeptics like caffeine, ephedrine, or amphetamines) to treat CNS depression, as this can exacerbate toxicity.
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.