1. Contact your veterinarian immediately.
  2. Your veterinarian may instruct you to induce vomiting, however, do not induce vomiting unless instructed to.
  3. Bring along the anti-histamine, with its package insert or label, that your pet has ingested. This will aid in confirmation of the specific drug ingested and calculation of dose and severity of the case.


Zyrtec , Telfast




Clinical signs vary according to the specific drug and dosage ingested and not all of the below clinical signs will be seen in each case.

  • Lethargy, Drowsiness
  • Sedation, depression
  • Nervousness/Hyperactivity
  • Uncoordinated/unsteady gait (ataxia)
  • Muscle tremors
  • Seizure
  • Coma
  • Nausea, Vomiting
  • Abdominal pain, gastrointestinal disturbances
  • Liver damage (rare)
  • Changes in heart rate and rhythm
  • Increased respiratory rate (tachypnoea)
  • Respiratory suppression, apnea
  • Cardiac arrest
  • Fever
  • Blindness, conjunctivitis, increased eye pressure
  • Fixed dilated pupils
  • Deafness
  • Urinary incontinence
  • Blood in urine
  • Urinary retention
  • Dry mucous membranes
  • Local irritation, itchy skin (pruritus), dry skin, hair loss


Large doses of antihistamines can lead to disruption of intestinal smooth muscle function causing gastrointestinal upsets. They can also suppress the respiratory system and impact on the nervous system, causing sedation or excitement.


Your veterinarian may use other methods of decontamination include gastric lavage to remove any remaining anti-histamines in your pet’s stomach. If vomiting is severe, an anti-emetic may be required. Activated charcoal can then be given to help absorb any remaining drugs in circulation. Intravenous fluid therapy may be required to help flush out the toxins, maintain normal blood pressure and hydration. Animals with acute overdose with anti-histamines often improve with treatment and supportive care within 24 hours although signs may persist for up to 3 days. Depending on the dose ingested and severity of clinical signs, animals that present seizing or in a coma have a more guarded prognosis as it is potentially life threatening and may require at least 1-2 weeks of supportive care.


Gfeller RW &Messonnier SP (1998) Handbook of Small Animal Toxicology & Poisonings. Missouri, USA: Mosby. pp. 82-84

Murphy L (2001) Antihistamine Toxicosis. Veterinary Medicine 96(10)

Bishoff K (2007) Toxicity of over-the-counter drugs. In Veterinary Toxicology. Ed. Gupta RC. USA: Elsevier. pp 363-390.