Diphenhydramine Intoxication in a Dog

Pharmacokinetics of diphenhydramine following single-dose intravenous and oral administration in non-fasted adult horses

Diphenhydramine is an H1 receptor antagonist used to control urticaria and other allergic signs caused by type I hypersensitivity reactions in horses (Equus caballus). Limited studies have been conducted on pharmacokinetics of this drug in horses, with no studies involving oral formulations. Our study investigated pharmacokinetics of an oral diphenhydramine formulation compared to intravenous administration in non-fasted adult horses. Six healthy horses underwent a single administration of three different doses of diphenhydramine (1 mg/kg intravenously, 1 mg/kg intragastrically, and 5 mg/kg intragastrically) with a two-week washout period between doses. Bioavailability of intragastric diphenhydramine was less than one percent and six percent for 1 mg/kg and 5 mg/kg intragastric doses, respectively. This poor bioavailability is similar to what is reported in dogs. Two of six horses experienced transient side effects after intravenous diphenhydramine administration, emphasizing the need for determining therapeutic plasma levels in efforts to determine the lowest effective dose minimizing risk of adverse effects. The main conclusion of our study was that oral diphenhydramine at doses up to 5 mg/kg are unlikely to achieve therapeutic plasma levels in adult horses.

The pharmacokinetics of DPH after the administration of a single intravenous or intramuscular dose in healthy dogs

The objective of this study was to determine the pharmacokinetics of diphenhydramine (DPH) in healthy dogs following a single i.v. or i.m. dose. Dogs were randomly allocated in two treatment groups and received DPH at 1 mg/kg, i.v., or 2 mg/kg, i.m. Blood samples were collected serially over 24 h. Plasma concentrations of DPH were determined by high-performance liquid chromatography, and noncompartmental pharmacokinetic analysis was performed with the commercially available software. Cardio-respiratory parameters, rectal temperature and effects on behaviour, such as sedation or excitement, were recorded. Diphenhydramine Clarea , Vdarea and T1/2 were 20.7 ± 2.9 mL/kg/min, 7.6 ± 0.7 L/kg and 4.2 ± 0.5 h for the i.v. route, respectively, and Clarea /F, Vdarea /F and T1/2 20.8 ± 2.7 mL/kg/min, 12.3 ± 1.2 L/kg and 6.8 ± 0.7 h for the i.m. route, respectively. Bioavailability was 88% after i.m. administration. No significant differences were found in physiological parameters between groups or within dogs of the same group, and values remained within normal limits. No adverse effects or changes in mental status were observed after the administration of DPH. Both routes of administration resulted in DPH plasma concentrations which exceeded levels considered therapeutic in humans.

Poisoning of dogs and cats by drugs intended for human use

One of the main causes of poisoning of small animals is exposure to drugs intended for human use. Poisoning may result from misuse by pet owners, off-label use of medicines or, more frequently, accidental ingestion of drugs that are improperly stored. This review focuses on classes of drugs intended for human use that are most commonly involved in the poisoning of small animals and provides an overview of poisoning episodes reported in the literature. To perform this review a comprehensive search of public databases (PubMed, Web of Science, Scopus, Google Scholar) using key search terms was conducted. Additionally, relevant textbooks and reference lists of articles pertaining to the topic were reviewed to locate additional related articles. Most published information on small animal poisoning by drugs intended for human use was from animal and human poison control centres or from single case reports. The dog was the species most frequently poisoned. The major drugs involved included analgesics (nonsteroidal anti-inflammatory drugs), antihistamines (H1-antihistamines), cardiovascular drugs (calcium channel blockers), central nervous system drugs (selective serotonin reuptake inhibitors, baclofen, benzodiazepines and zolpidem), gastrointestinal drugs (loperamide), nutritional supplements (vitamin D and iron salts) and respiratory drugs (β2-adrenergic receptor agonists).