Diphenhydramine
Diphenhydramine is a **first-generation ethanolamine-derivative antihistamine** widely used in veterinary medicine. * **Primary Indications**: Management of allergic reactions (urticaria, angioedema, atopy), pruritus, and anaphylaxis. * **Secondary Indications**: Prevention of motion sickness, mild sedation, and as an antiemetic. **Clinical Pearls**: * Because it readily crosses the blood-brain barrier, it causes more pronounced CNS depression (sedation) compared to second-generation antihistamines (e.g., cetirizine, loratadine). * In cats, histamine is not considered a primary mediator of emesis. Therefore, NK-1 antagonists (like maropitant) or M1-cholinergic antagonists are generally preferred over diphenhydramine for treating feline motion sickness or vomiting. * It is sometimes used as an adjunctive treatment for aseptic laminitis in cattle and feline pancreatitis.
Mecanismo: Diphenhydramine exerts its effects through multiple pathways: * **H1-Receptor Antagonism**: Competitively binds to and blocks **H1 receptors** on effector cells → prevents histamine-mediated capillary permeability, vasodilation, and smooth muscle spasms (bronchoconstriction). * **Central Effects**: Readily crosses the blood-brain barrier → blocks central **H1 receptors** → produces sedation and antiemetic effects. * **Anticholinergic Activity**: Non-selectively binds to **muscarinic receptors** → decreases exocrine secretions (drying effect) and can cause urinary retention or tachycardia. * **Additional Effects**: Possesses mild antitussive properties.
Dosificación por especie
- As an antihistamine · 0.5 mg/kg · PO · q12h · Liquid formulation is distasteful
- As an antihistamine · 2-4 mg (total dose) · PO · q12-24h
- As an antihistamine · 2-4 mg/kg · PO · q8h
- For severe urticaria and angioedema · 2 mg/kg · IM · twice daily as needed · Used with steroids (prednisone 2 mg/kg IM twice daily) and epinephrine 1:10,000 (0.5-2 mL SC)
- For adjunctive treatment of pancreatitis · 2-4 mg/kg · PO · q8h
- Prevaccination · 2 mg/kg · PO, IM or IV · 10 minutes prior to vaccination
- Pretreatment before doxorubicin · 5 mg (total dose) · IM · Single dose
- For adjunctive therapy of anaphylaxis · 0.5-1 mg/kg · IM or IV · Single dose · Used with epinephrine and steroids
- For adjunctive therapy of aseptic laminitis · 55-110 mg/100 kg body weight (0.55-1.1 mg/kg) · IV or IM · Single dose · During the acute phase · Used with corticosteroids
Vías de administración
Contraindicaciones
- Hypersensitivity to diphenhydramine or other antihistamines in its class
- Angle closure glaucoma
- Prostatic hypertrophy
- Pyloroduodenal or bladder neck obstruction
- COPD (if mucosal secretions are a problem)
- Hyperthyroidism (use with caution)
- Cardiovascular disease or hypertension (use with caution)
- Seizure disorders (use with caution)
- Known hypersensitivity to diphenhydramine
- Glaucoma (due to anticholinergic effects)
- Prostatic hypertrophy or urinary retention
- Gastrointestinal obstruction
- Use with caution in working dogs due to sedation
Efectos adversos
- CNS depression (lethargy, somnolence)
- Anticholinergic effects (dry mouth, urinary retention)
- GI effects (diarrhea, vomiting, anorexia)
- Paradoxical excitement (especially in cats)
- Sedation/lethargy
- Dry mouth (xerostomia)
- Urinary retention
- Tachycardia
- Gastrointestinal disturbances (diarrhea, vomiting, anorexia)
Interacciones farmacológicas
- Anticholinergic drugs (including tricyclic antidepressants) · May potentiate anticholinergic effects
- CNS depressant drugs · Increased sedation can occur
- CNS Depressants (e.g., opioids, benzodiazepines, barbiturates) · Additive sedation and central nervous system depression · moderate
- Anticholinergic drugs · Additive anticholinergic effects (dry mouth, tachycardia, urinary retention) · moderate
- Monoamine Oxidase Inhibitors (MAOIs) · May prolong and intensify the anticholinergic effects of antihistamines · major
- CNS Depressants (e.g., Diazepam, Phenobarbital, Gabapentin) · Additive CNS depression and sedation · moderate
- Anticholinergic drugs (e.g., Atropine, Glycopyrrolate) · Additive anticholinergic effects (tachycardia, dry mouth, ileus) · moderate
- Tricyclic Antidepressants (e.g., Amitriptyline) · Increased risk of anticholinergic toxicity and sedation · moderate
Monitorización
- Clinical efficacy (reduction in pruritus, allergic signs, or vomiting)
- Adverse effects (excessive sedation, anticholinergic signs)
- Resolution of allergic signs or pruritus
- Degree of sedation
- Heart rate (monitor for tachycardia)
- Urination frequency (monitor for retention)
Sobredosis
Overdosage can cause **CNS stimulation** (ranging from excitement to seizures) or **CNS depression** (lethargy to coma), severe anticholinergic effects, respiratory depression, and death. **Treatment**: * Empty the gut after oral ingestion using standard protocols. * Induce emesis if the patient is alert and CNS status is stable. * Administer a saline cathartic and/or activated charcoal after emesis or gastric lavage. * Provide symptomatic and supportive therapies. * **Seizure management**: Phenytoin (IV) is recommended in humans for seizures caused by antihistamine overdose; **barbiturates and diazepam should be avoided**.
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