ジフェンヒドラミン
ジフェンヒドラミンは、獣医学で広く使用されている**第一世代エタノールアミン系抗ヒスタミン薬**です。 * **主な適応症**：アレルギー反応（蕁麻疹、血管浮腫、アトピー）、瘙痒症、およびアナフィラキシーの管理。 * **二次的な適応症**：乗り物酔いの予防、軽度の鎮静、および制吐薬として。 **臨床上のポイント**： * 血液脳関門を容易に通過するため、第二世代抗ヒスタミン薬（セチリジン、ロラタジンなど）と比較して、より顕著な中枢神経抑制（鎮静）を引き起こします。 * 猫において、ヒスタミンは嘔吐の主要なメディエーターではないと考えられています。そのため、猫の乗り物酔いや嘔吐の治療には、ジフェンヒドラミンよりもNK-1受容体拮抗薬（マロピタントなど）やM1コリン受容体拮抗薬が一般的に好まれます。 * 牛の無菌性蹄葉炎や猫の膵炎の補助治療として使用されることもあります。
作用機序: 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.
動物種別の用量
- 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
投与経路
禁忌
- 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
有害事象
- 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)
薬物相互作用
- 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
モニタリング
- 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)
過量投与
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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