Diphenhydramine

Overview

Diphenhydramine is a first-generation antihistamine (H1 receptor antagonist) commonly used in dogs and cats for its antihistaminic, antiemetic, and sedative effects. It is widely used in veterinary medicine for the management of allergic and pruritic conditions.

In addition to its antihistaminic action, diphenhydramine has anticholinergic and central nervous system depressant effects, which contribute to its clinical uses such as motion sickness prevention and mild sedation.

Mechanism of Action (MOA): Diphenhydramine competitively blocks H1 histamine receptors, reducing the effects of histamine release such as pruritus, vasodilation, and increased vascular permeability. It also exhibits anticholinergic and CNS depressant activity, contributing to its antiemetic and sedative properties.

Indications

Diphenhydramine is used in dogs and cats primarily for its antihistaminic effects in the management of allergic conditions. Additional uses include antiemetic and adjunctive therapies in selected cases.

• Allergic and pruritic conditions: Used for management of urticaria, angioedema, insect bite hypersensitivity, and other allergic reactions.
• Adjunct in anaphylaxis and mast cell disease: May be used to help reduce effects of histamine release, including perioperative use in mast cell tumor patients.
• Atopic dermatitis (limited efficacy): May provide benefit in mild cases or as part of preventive strategies, although response is variable.
• Motion sickness and antiemetic use: Used for prevention of motion sickness and control of vomiting, although efficacy may be limited, particularly in cats.
• Sedation (adjunctive use): May produce mild sedation, especially when administered orally.

Dosage (Reference)

Dog

In dogs, diphenhydramine is administered orally or parenterally depending on clinical indication. Dose selection depends on the route and desired effect.

Cat

In cats, diphenhydramine is used similarly to dogs for allergic conditions, although response may be variable and adverse behavioral effects may occur.

Warnings & Precautions

Diphenhydramine should be used cautiously in dogs and cats due to its anticholinergic and central nervous system effects. Patient selection and monitoring are important to minimize adverse outcomes.

• Contraindication (hypersensitivity): Do not use in patients with known hypersensitivity to diphenhydramine or related antihistamines.
• Anticholinergic effects: Use cautiously in patients with angle-closure glaucoma, urinary retention, or gastrointestinal obstruction.
• Respiratory disease: Use with caution in patients with chronic obstructive pulmonary disease, as thickened secretions may worsen clinical signs.
• Cardiovascular and endocrine disease: Use cautiously in patients with hyperthyroidism, cardiovascular disease, or hypertension.
• Neurologic disorders: May lower seizure threshold or exacerbate neurologic conditions; use cautiously in patients with seizure disorders.
• Performance animals: Sedative effects may impair working or performance dogs.
• Use in young animals: Avoid use in neonates due to increased susceptibility to CNS adverse effects.
• Drug confusion risk: Do not confuse diphenhydramine with similarly named drugs (e.g., dimenhydrinate).

Drug Interactions

Clinically relevant interactions with diphenhydramine are primarily related to its anticholinergic and central nervous system depressant effects. Careful monitoring and dose adjustment may be required when used with the following medications.

• Acetylcholinesterase inhibitors (e.g., neostigmine, pyridostigmine): Diphenhydramine may reduce their therapeutic efficacy.
• Anticholinergic drugs (e.g., atropine, glycopyrrolate): Additive anticholinergic effects such as dry mucous membranes, constipation, tachycardia, and urinary retention.
• CNS depressants (e.g., anesthetics, barbiturates, benzodiazepines, opioids): Increased CNS and respiratory depression; lower doses may be required.
• Monoamine oxidase inhibitors (e.g., selegiline, amitraz): May potentiate anticholinergic effects.
• Potassium supplements (oral): Reduced GI motility may increase risk of gastrointestinal irritation or ulceration, especially with solid formulations.
• Prokinetic agents (e.g., metoclopramide, cisapride): May reduce the therapeutic effect of prokinetic drugs.
• Tricyclic antidepressants (e.g., amitriptyline, clomipramine): Increased risk of CNS depression and additive anticholinergic effects.

Side Effects & Overdose

Side Effects

Adverse effects of diphenhydramine are primarily related to central nervous system depression and anticholinergic activity. Effects may be dose-dependent and vary between species.

• CNS depression: Lethargy and sedation are the most common effects.
• Anticholinergic effects: Dry mouth, constipation, and urinary retention may occur.
• Gastrointestinal signs: Vomiting, diarrhea, and anorexia may be observed.
• Paradoxical excitation (cats): Cats may develop agitation or excitatory behavior instead of sedation.
• Local reactions: Subcutaneous injection may cause tissue irritation or necrosis.

Overdose

Overdose can result in significant neurologic, cardiovascular, and anticholinergic toxicity.

• CNS effects: Excitation (e.g., agitation, seizures) or depression (e.g., coma).
• Anticholinergic toxicity: Tachycardia, dry mucous membranes, and decreased gastrointestinal motility.
• Cardiovascular abnormalities: May include arrhythmias or blood pressure changes.
• Respiratory depression: May occur in severe cases.
• Management: GI decontamination (if appropriate) and supportive care; monitor closely and treat based on clinical signs.

Key Notes

Practical clinical considerations for the use of diphenhydramine in dogs and cats:

• Variable clinical response: Efficacy as an antihistamine is inconsistent, and trial periods (1–2 weeks) are often needed to assess response.
• Better for prevention than treatment: May be more effective when given before allergen exposure rather than after clinical signs develop.
• Bioavailability differences: Oral bioavailability in dogs is relatively low compared to parenteral administration.
• Combination products: Formulations such as dimenhydrinate may alter pharmacokinetics and increase systemic exposure.
• Formulation considerations: Some liquid products may contain alcohol or additional active ingredients; ensure product selection is appropriate.