ヒドロキシジン
ヒドロキシジンは、獣医療で広く使用されている第一世代のピペラジン系抗ヒスタミン薬です。主に抗そう痒作用および抗ヒスタミン作用を目的として処方され、犬や猫のアトピー性皮膚炎、ノミアレルギー性皮膚炎などのアレルギー疾患の管理に頻用されます。 血液脳関門を通過するため、鎮静作用や精神安定作用も示し、不安を抱える動物や激しい痒みから自傷行為に及ぶ動物に非常に有効です。猫では、その鎮痙作用と軽度の抗不安作用により、頻回再発性の特発性下部尿路疾患（FLUTD）の管理に使用されることもあります。
作用機序: Hydroxyzine acts as a competitive antagonist at **H1-receptors** on effector cells. * **Histamine Blockade**: Competes with histamine for H1-receptor sites → prevents histamine-induced bronchoconstriction, vasodilation, and capillary permeability. *Note: It does not inhibit the release of histamine from mast cells, but antagonizes its effects.* * **CNS Effects**: Readily crosses the blood-brain barrier → binds to central H1 receptors → causes sedation and tranquilization. * **Additional Properties**: Exhibits anticholinergic, antispasmodic, local anesthetic, mild bronchodilative, and antiemetic activities. * **Metabolism Pearl**: In vivo, hydroxyzine is metabolized in the liver to its active metabolite, **cetirizine**, which contributes significantly to its prolonged antihistaminic effects.
動物種別の用量
- For pruritus · 1-2 mg/kg or 5-10 mg/cat PO q8-12h · PO · q8-12h
- For pruritus · 5-10 mg (total dose) or 2.2 mg/kg PO q8-12h · PO · q8-12h
- For frequently recurrent idiopathic lower urinary tract disease · 5-10 mg (total dose) per cat PO q12h · PO · q12h
- General · 2 mg/kg PO 3 times daily · PO · q8h
- General · 0.5-1 mg/kg IM or PO twice daily · IM/PO · q12h · ARCI UCGFS Class 2 Drug
- General · 0.67 mg/kg PO twice daily · PO · q12h · Using the pamoate salt
- For pruritus associated with allergies, feather picking, or self-mutilation · 2 mg/kg q8h PO or 1.5-2 mg per 4 oz of drinking water daily · PO · q8h or daily · Adjust dose to minimize drowsiness and maximize effect
- For pruritus associated with allergies, feather picking, or self-mutilation · 2 mg/kg PO q12h · PO · q12h
- As an antipruritic/antihistamine · 2.2 mg/kg PO three times daily (q8h) · PO · q8h
投与経路
禁忌
- Hypersensitivity to hydroxyzine or its metabolites (e.g., cetirizine)
- Known hypersensitivity to hydroxyzine or cetirizine
- Early pregnancy (teratogenic in some animal models)
有害事象
- Sedation (most common, usually mild and transient in dogs)
- Hyperexcitability (occasional paradoxical reaction)
- Fine rapid tremors (dogs)
- Whole body tremors (dogs)
- Seizures (rare in dogs)
- Polydipsia (cats)
- Depression (cats)
- Behavioral changes (cats)
- Sedation / Lethargy
- Dry mouth
- Urinary retention
- Gastrointestinal upset
- Paradoxical excitation (rare)
薬物相互作用
- Anticholinergic agents · Additive anticholinergic effects may occur when used concomitantly. · moderate
- CNS Depressant Drugs (e.g., barbiturates, tranquilizers) · Additive CNS depression may be seen.
- Epinephrine · Hydroxyzine may inhibit or reverse the vasopressor effects of epinephrine; use norepinephrine or metaraminol instead.
- CNS Depressants (e.g., opioids, barbiturates, gabapentin) · Additive CNS depression and sedation · major
モニタリング
- Efficacy (reduction in pruritus or clinical signs)
- Adverse effects (degree of sedation, excitability, or behavioral changes)
- Resolution of pruritus or allergic signs
- Degree of sedation
- Adverse anticholinergic effects (e.g., monitoring urination)
過量投与
Overdoses would be expected to cause **increased sedation** and perhaps **hypotension**. At high doses, fine rapid tremors and rarely seizures have been reported. Common clinical signs of toxicity in dogs include: * Hyperthermia * Lethargy * Tachycardia * Trembling / Tremors * Ataxia * Somnolence **Treatment**: There are no specific antidotes available. Gut emptying protocols should be considered with large or unknown quantity overdoses. Supportive and symptomatic treatment is recommended if necessary.
VetSheet の薬剤リファレンスは、獣医療従事者向けの臨床意思決定支援を目的としており、専門的判断やメーカーの最新添付文書に代わるものではありません。