クロルフェニラミン
クロルフェニラミンは第一世代のアルキルアミン系抗ヒスタミン薬であり、獣医療では主に抗ヒスタミン作用および止痒作用を目的として使用されます。 **臨床上のポイント:** * **第一世代抗ヒスタミン薬:** 血液脳関門を通過するため、中枢神経抑制（鎮静）を引き起こすことが多く、軽度の鎮静剤として治療に利用されることもあります。 * **猫のそう痒症:** アレルギー性皮膚疾患やそう痒の管理において、猫で最もよく使用される抗ヒスタミン薬の一つです。 * **肥満細胞腫:** 全身的なヒスタミン放出の影響を遮断するため、肥満細胞腫の化学療法プロトコルにおける補助療法としてしばしば使用されます。 * **奇異反応（興奮）:** 犬では通常鎮静を引き起こしますが、猫では逆に興奮状態を示すことがあります。
作用機序: Chlorpheniramine acts as a competitive **H1-receptor antagonist**. * **Histamine Blockade:** It competitively inhibits histamine from binding to **H1 receptors** on effector cells (such as smooth muscle and endothelial cells) → prevents histamine-induced vasodilation, increased capillary permeability, and smooth muscle spasms. * **Note:** It does *not* inactivate histamine or prevent its release from mast cells; it only blocks its action at the receptor site. * **Additional Properties:** Like many first-generation antihistamines, it possesses varying degrees of **anticholinergic** (muscarinic blockade) and **CNS depressant** activity.
動物種別の用量
- General antihistamine use · 2 mg (total dose) per cat PO every 12 hours · PO · q12h
- General antihistamine use · 2-4 mg per cat q12-24h PO · PO · q12-24h
- General antihistamine use · 2 mg per cat two to three times daily · PO · BID-TID · Most common dosage in cats
- Pruritus · 2-4 mg/cat twice daily; rarely may be maintained on once daily dosing. · PO · BID · Palatability may be enhanced by dipping the split tablet into tuna fish 'juice', butter or petrolatum; placing split tablets into empty gelatin capsules or sprinkling or mixing timed release beads (partial contents of an 8 mg capsule) with food.
- Pruritus · 0.5-2 mg (total dose; ¼ - ½ of a 4 mg tablet) PO 2-3 times per day · PO · BID-TID
- Mild sedative · 1-2 mg per cat q12-24h (low dose), 2-4 mg per cat PO q12-24h (high dose) · PO · q12-24h
- General antihistamine use · 1-2 mg/kg PO 2-3 times a day · PO · BID-TID
- General use / Feather damaging behavior · One 4 mg tablet in one cup (240 mL; 8 oz.) of bottled water to be used as drinking water; changed daily. · PO · q24h
投与経路
禁忌
- Hypersensitivity to chlorpheniramine or other alkylamine antihistamines
有害事象
- CNS depression (lethargy, somnolence, sedation)
- GI effects (diarrhea, vomiting, anorexia)
- Anticholinergic effects (dry mouth, urinary retention)
- Paradoxical excitement (especially in cats)
- Decreased performance in working dogs due to sedation
薬物相互作用
- Anticoagulants (heparin, warfarin) · Antihistamines may partially counteract the anticoagulation effects of heparin or warfarin.
- MAO Inhibitors (amitraz, selegiline) · May prolong and exacerbate the anticholinergic effects of the antihistamine.
- CNS Depressants · Increased sedation and CNS depression can occur when used concurrently.
- CNS Depressants (e.g., opioids, sedatives, barbiturates) · Additive CNS depression and sedation · moderate
- Anticholinergic drugs · Additive anticholinergic effects (dry mouth, tachycardia, urinary retention) · moderate
モニタリング
- Clinical efficacy (reduction in pruritus, allergic signs, or adequate sedation)
- Adverse effects (excessive sedation, GI upset, anticholinergic signs)
過量投与
### Overdose Signs Overdosage may cause **CNS stimulation** (ranging from excitement to seizures) or **CNS depression** (lethargy to coma), anticholinergic effects, respiratory depression, and death. > *Case report:* A 9-month-old dachshund ingesting 25 mg/kg showed signs of ataxia, tremors, bradycardia, coma, and cardiac arrest, dying within 11 hours of ingestion. ### Treatment * **Decontamination:** Empty the gut using standard protocols if ingestion was oral. Induce emesis if the patient is alert and CNS status is stable. Follow with a saline cathartic and/or activated charcoal. * **Supportive Care:** Treat other clinical signs symptomatically. * **Seizure Control:** **Phenytoin (IV)** is recommended in the treatment of seizures caused by antihistamine overdoses in humans; **barbiturates and diazepam should be avoided**.
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。