VetSheet

Pyrilamine Maleate

Pyrilamine maleate (Mepyramine maleate)

Antihistamine (First-Generation, Ethylenediamine class)POIMIVSCDogsHorsesCattleSwineSheep

Also known as: Histall · Equiphed · Anihist · Alergitanil · Antemesyl · Anthisan · Equi-Phar Equi-Hist · Fluidasa · Histagranules · Histamed · Mepyraderm · Mepyrimal · Pyriped · Relaxa-Tabs · TriHist · pyranisamine hydrochloride · pyrilamine hydrochloride · mepyramine hydrochloride · mepyramini maleas · myranisamine maleate · myrilamine maleate · mepyramine maleate

Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference

12.5-25 mg PO four times a dayPO· q6h
12.5–25 mg — Fixed dose per animal (not weight-scaled)

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

IndicationDoseRouteFrequencyDurationRegion
Antihistamine12.5-25 mg PO four times a dayPOq6h—🌎 NA
Antihistamine25-125 mg IMIMUnknown—🌎 NA

Horses

IndicationDoseRouteFrequencyDurationRegion
Antihistamine0.88-1.32 mg/kg (2-3 mL of 20 mg/mL solution per 100 lbs body weight) IV (slowly), IM or SCIV, IM, SCq6-12h—🌎 NA
Antihistamine (Foals)0.44 mg/kg (1 mL of 20 mg/mL solution per 100 lbs. body weight) IV (slowly), IM or SCIV, IM, SCq6-12h—🌎 NA
Antihistamine1 mg/kg IV , IM or SCIV, IM, SCUnknown—🌎 NA
Antihistamine0.5-1.5 grams IMIMUnknown—🌎 NA
  • Antihistamine: May repeat in 6-12 hours if necessary. NOTE: ARCI UCGFS CLASS 3 DRUG
  • Antihistamine (Foals): May repeat in 6-12 hours if necessary.

Cattle

IndicationDoseRouteFrequencyDurationRegion
Antihistamine0.5-1.5 grams IMIMUnknown—🌎 NA
Adjunctive treatment of aseptic laminitis55-110 mg/100 kg IV or IMIV, IMUnknown—🌎 NA

Swine

IndicationDoseRouteFrequencyDurationRegion
Antihistamine0.25-0.5 gram IMIMUnknown—🌎 NA

Sheep

IndicationDoseRouteFrequencyDurationRegion
Antihistamine0.25-0.5 gram IMIMUnknown—🌎 NA

Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.

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Overview

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 of action

Pyrilamine acts as a ​competitive antagonist (inverse agonist)​ at H1-receptors.

  • Mechanism: Allergic trigger → Mast cell degranulation → 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.

Safety & warnings

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

Precautions

Emergency Use Warning: Antihistamines are not a substitute for epinephrine and supportive care in cases of severe, acute anaphylaxis.

  • Skin Testing: Antihistamines suppress the wheal and flare response. Discontinue at least 4 days prior to intradermal allergy testing.
  • Pregnancy: Probably safe at usual doses, though high-dose rodent studies showed increased fetal death. No teratogenic effects noted in human studies.
  • Nursing: Unknown if it enters milk.
  • Regulatory: Products containing pseudoephedrine are subject to strict regulations (List 1 chemical) due to illicit methamphetamine manufacturing. Pyrilamine is an ARCI Class 3 drug in racing horses.

Drug interactions

Anticoagulants (heparin, warfarin)

Antihistamines may partially counteract the anticoagulation effects of heparin or warfarin.

CNS Depressant Drugs

Increased sedation can occur if pyrilamine is combined with other CNS depressant drugs.

Epinephrine

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)

Pharmacokinetics

Half-life

Horses1.7 hours (IV)

Absorption

In horses, pyrilamine is poorly bioavailable (18%) after oral administration.

Distribution

Not extensively studied in cattle, dogs, or cats.

Metabolism

The principle metabolite is O-desmethylpyrilamine (O-DMP).

Elimination

After a single dose, O-DMP can be detected in urine for at least two days, and possibly up to one week after dosing.

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.

Available products

Formulations

  • Granules
  • Injection
  • Oral Syrup (combination products)

Veterinary

  • Pyrilamine Granules: 600 mg/oz (Histall)
  • Pseudoephedrine HCl 600 mg/oz and Pyrilamine maleate 600 mg/oz Granules (Equiphed, Equi-Phar Equi-Hist 1200 Granules, Tri-Hist Granules, Histagranules)
  • Pyrilamine 600 mg/oz and Guaifenesin 2400 mg/oz Granules (Anihist, Hist-EQ)
  • Combination cough syrups containing pyrilamine labeled for use in small animals

Regulatory status

No regulatory data for: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU

Storage & stability

Avoid freezing the injectable product.

Client information

Pyrilamine is an antihistamine used primarily to treat allergic reactions, hives, and respiratory allergies in horses.

  • What to Expect: It helps reduce swelling, itching, and allergic airway responses.
  • Side Effects: Watch your animal closely for changes in behavior. Some animals may become unusually sleepy (sedated), while others—especially horses—might become nervous, excitable, or uncoordinated. Contact your veterinarian if you notice muscle weakness, tremors, or loss of appetite.
  • Competition Horses: If your horse competes, be aware that this medication is regulated (ARCI Class 3). Do not use it within 72 hours of a sporting event, and consult your governing body's rules regarding withdrawal times.
  • Allergy Testing: If your pet is scheduled for skin allergy testing, this medication must be stopped at least 4 days prior to the test.

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.