氯雷他定
**氯雷他定 (Loratadine)** 是一種第二代、無鎮靜作用的 H1 受體拮抗劑，在獸醫學中主要用於控制過敏性疾病，如異位性皮膚炎、疫苗反應和昆蟲叮咬。與第一代抗組織胺（如苯海拉明）不同，氯雷他定不易穿過血腦屏障，大幅降低了中樞神經系統抑制和鎮靜的發生率。 > **臨床警告：** 務必確保人類非處方氯雷他定產品不含解充血劑（如**偽麻黃鹼**，通常標示為 'Claritin-D'），因為偽麻黃鹼對犬貓具有高度毒性，可能導致致命的心血管和神經刺激。
作用機制: Loratadine acts as a selective inverse agonist/antagonist at peripheral **H1 histamine receptors**. * **Histamine Blockade:** It competitively binds to H1 receptors on effector cells in the gastrointestinal tract, blood vessels, and respiratory tract → prevents histamine from binding and exerting its pro-inflammatory and vasodilatory effects. * **Non-sedating:** Due to its lipophobic nature and large molecular size, it has poor penetration across the blood-brain barrier, resulting in minimal to no central H1 receptor blockade.
給藥途徑
禁忌症
- Known hypersensitivity to loratadine or desloratadine
- Use with caution in patients with severe hepatic impairment
- Use with caution in patients with keratoconjunctivitis sicca (dry eye)
不良反應
- Mild lethargy or sedation (rare compared to first-generation antihistamines)
- Vomiting
- Diarrhea
- Dry mouth (xerostomia)
- Decreased tear production
藥物相互作用
- Ketoconazole · May inhibit the metabolism of loratadine, increasing plasma concentrations · moderate
- Erythromycin · May inhibit the metabolism of loratadine, increasing plasma concentrations · moderate
- Cimetidine · May inhibit the metabolism of loratadine, increasing plasma concentrations · moderate
- Amiodarone · May increase loratadine levels via CYP inhibition · moderate
監測
- Resolution of allergic clinical signs (e.g., reduced pruritus, erythema)
- Adverse gastrointestinal effects
- Tear production (Schirmer Tear Test) if dry eye is suspected
過量
Loratadine has a wide margin of safety. Massive overdosage may cause tachycardia, somnolence, and headache. **Treatment:** Consists of standard gastrointestinal decontamination (emesis induction, activated charcoal) if ingestion is recent, followed by symptomatic and supportive care. Loratadine is not significantly removed by hemodialysis.
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