フェニトインナトリウム
Phenytoin Sodium
Diphenylhydantoin
別名: Dilantin · Phenytek · Diphenylhydantoin · DPH · Fenitoina · Phenantoinum · Phenytoinum
VetSheet 獣医チームが監修更新日 2026年4月5日エビデンスに基づく獣医学リファレンス
用量は獣医専門家向けの臨床参考資料です。必ず最新の医薬品情報および個別症例に基づいて確認してください。
動物種別用量
🌎 NA — North America🌍 EU — Europe
犬
| 適応 | 用量 | 経路 | 頻度 | 期間 | 地域 |
|---|---|---|---|---|---|
| Treatment of seizures | 15-40 mg/kg | PO | three times daily | — | 🌎 NA |
| Treatment of seizures | 20-35 mg/kg | PO | three times daily | — | 🌎 NA |
| Treatment of seizures | 8.8-17.6 mg/kg in divided doses | PO | divided doses | — | 🌎 NA |
| Treatment of ventricular arrhythmias | Up to 10 mg/kg in increments of 2-4 mg/kg OR 20-35 mg/kg | IV or PO | three times daily (for PO) | — | 🌎 NA |
| Treatment of ventricular arrhythmias | 10 mg/kg slowly IV; 30-50 mg/kg PO | IV, PO | q8h (for PO) | — | 🌎 NA |
| Treatment (or prophylaxis) of digitalis intoxication | 50 mg/kg | PO | q8h | — | 🌎 NA |
| Treatment of hypoglycemia secondary to tumor | 6 mg/kg | PO | two to three times daily | — | 🌎 NA |
- Treatment of seizures: Gradually increase or decrease dose to maintain control. May take several days for seizure control. Note: Unlikely to attain necessary serum levels due to fast half-life.
- Treatment (or prophylaxis) of digitalis intoxication: Long-term use may cause increases in serum alkaline phosphatase and increased hepatic cell size.
猫
| 適応 | 用量 | 経路 | 頻度 | 期間 | 地域 |
|---|---|---|---|---|---|
| Treatment of ventricular arrhythmias | 2-3 mg/kg | PO | q24h | — | 🌎 NA |
| Treatment of seizures | 2-3 mg/kg daily; 20 mg/kg per week | PO | daily or weekly | — | 🌎 NA |
- Treatment of ventricular arrhythmias: Diligent monitoring is required due to accumulation risk.
- Treatment of seizures: Use is very controversial in this species.
馬
| 適応 | 用量 | 経路 | 頻度 | 期間 | 地域 |
|---|---|---|---|---|---|
| Seizures | 2.83-16.43 mg/kg | PO | q8h | — | 🌎 NA |
| Digoxin induced arrhythmias | 10-22 mg/kg | PO | q12h | — | 🌎 NA |
| Treatment of ventricular dysrhythmias | 20 mg/kg initially for the first 3-4 doses, followed by a maintenance dose of 10-15 mg/kg | PO | q12h | — | 🌎 NA |
- Seizures: To obtain serum levels from 5-10 micrograms/mL. Suggest monitoring serum levels to adjust dosage.
- Digoxin induced arrhythmias: Adverse effects are muscle fasciculations and sedation.
- Treatment of ventricular dysrhythmias: For persistent ventricular extra systoles or ventricular tachycardia where conventional treatment has failed. Suggest monitoring plasma concentrations.
用量は獣医専門家向けの臨床参考資料です。必ず最新の医薬品情報および個別症例に基づいて確認してください。
概要
フェニトインナトリウムは、ヒダントイン誘導体の抗けいれん薬およびクラスIB抗不整脈薬です。ヒト医療においては歴史的に重要ですが、伴侶動物における薬物動態プロファイルが好ましくないため、獣医療での使用は非常に制限されています。
- 犬：半減期が非常に短く、急速な肝酵素の自己誘導が起こるため、頻回かつ高用量の投与を行わない限り治療血清濃度を維持することがほぼ不可能であり、長期的なてんかん管理にはほとんど使用されません。
- 猫：半減期が著しく延長（最大108時間）し、グルクロン酸抱合経路が欠如しているため、毒性の蓄積に対して非常に敏感です。
- 臨床的適応：現在では主に、犬や馬におけるジゴキシン誘発性心室性不整脈の治療、または猫の特定のまれな神経筋疾患（ミオキミアや神経筋緊張症など）に限定して使用されます。
臨床のポイント：インスリン分泌を抑制する作用があるため、インスリノーマに続発する低血糖の補助治療として研究されてきましたが、臨床的な効果は通常わずかです。
作用機序
Phenytoin exhibits both neurological and cardiac effects through the stabilization of excitable membranes:
- Neurological: Binds to and prolongs the inactive state of voltage-gated sodium channels (VGSCs) → promotes sodium efflux and limits sodium influx → stabilizes neuronal membranes and prevents the high-frequency repetitive firing necessary for seizure propagation from epileptogenic foci.
- Cardiac: Acts as a Class IB antiarrhythmic (similar to lidocaine) → slightly depresses phase 0 of the cardiac action potential and shortens phase 3 repolarization → decreases overall action potential duration. It is particularly effective against digitalis-induced arrhythmias.
- Endocrine: Inhibits the secretion of insulin and vasopressin (ADH).
安全性・警告
禁忌
- Hypersensitivity to phenytoin or other hydantoins
- Intravenous use is contraindicated in 2nd or 3rd degree heart block
- Sinoatrial block
- Adams-Stokes syndrome
- Sinus bradycardia
有害事象
- Dogs: Anorexia, vomiting, ataxia, sedation, gingival hyperplasia, hepatotoxicity (elevated ALT, decreased albumin, hepatic lipidosis)
- Cats: Ataxia, sedation, anorexia, dermal atrophy syndrome, thrombocytopenia
- Horses: Excitement, recumbency (at high plasma concentrations)
注意事項
Hepatotoxicity Warning: Chronic therapy in dogs requires regular monitoring of liver function due to the risk of hepatocellular hypertrophy, necrosis, and hepatic lipidosis.
- Pregnancy: Potentially teratogenic (FDA Category D / Papich Class C). It readily crosses the placenta and is excreted in maternal milk. Use only when benefits clearly outweigh risks.
- Feline Sensitivity: Cats metabolize phenytoin extremely slowly. Use is highly controversial and requires diligent monitoring to prevent severe toxicity.
- IV Administration: Must be given slowly. Rapid IV injection can cause severe hypotension and cardiac arrhythmias.
医薬品相互作用
Significantly increases the serum half-life of phenytoin (e.g., from 3 to 15 hours in dogs) by inhibiting its hepatic metabolism.
The toxicity of lithium may be enhanced.
Phenytoin may decrease the analgesic properties of meperidine but enhance its toxic effects.
Altered pharmacologic effects; potential for additive hepatotoxicity. Weigh risks vs. benefits before combining.
May increase the pharmacologic effects and toxicity of phenytoin.
May decrease the pharmacologic activity of phenytoin.
Phenytoin may decrease the pharmacologic activity of these agents via hepatic enzyme induction.
監視
- Level of seizure control or arrhythmia resolution
- Signs of toxicity (sedation, ataxia)
- Body weight (monitor for anorexia)
- Liver enzymes (ALT, ALP) and serum albumin (especially with chronic therapy)
- Serum drug levels (if signs of toxicity appear or lack of efficacy is noted)
薬物動態
半減期
吸収
Nearly completely absorbed in humans, but in dogs, oral bioavailability may only be about 40%.
分布
Well distributed throughout the body. About 78% bound to plasma proteins in dogs (vs. 95% in humans). Protein binding may be reduced in uremic patients. Readily crosses the placenta and small amounts are excreted into milk.
代謝
Metabolized in the liver; much of the drug is conjugated to a glucuronide form. Phenytoin induces hepatic microsomal enzymes, enhancing the metabolism of itself (autoinduction) and other drugs.
消失
Excreted by the kidneys as metabolites.
過剰投与
Clinical signs of overdosage are dose-dependent:
- Lower levels: Sedation, anorexia, and ataxia (wobbly gait).
- Higher levels: Coma, severe hypotension, and respiratory depression.
Treatment: Dogs rapidly clear the drug, so treatment depends on the severity of clinical signs. Severe intoxications require aggressive supportive care (IV fluids, cardiovascular and respiratory support).
製品
製剤
- Extended-release capsules: 30 mg, 100 mg, 200 mg, 300 mg
- Oral suspension: 25 mg/mL
- Chewable tablets: 50 mg
- Injection: 50 mg/mL
動物用医薬品
- None currently available
ヒト用医薬品
- Phenytoin Sodium Extended-Release Capsules: 30 mg, 100 mg, 200 mg & 300 mg (Dilantin Kapseals, Phenytek)
- Phenytoin Oral Suspension: 25 mg/mL (Dilantin-125)
- Phenytoin Tablets: 50 mg chewable (Dilantin Infa-Tabs)
- Phenytoin Sodium Injection: 50 mg/mL (46 mg/mL phenytoin base)
規制ステータス
規制データなし: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
保管・安定性
Store capsules at room temperature (below 86°F) and protect from light and moisture. Store injection at room temperature and protect from freezing. If injection is frozen/refrigerated, a precipitate may form which should resolubilize when warmed. Do not use precipitated solutions. Injectable solutions will precipitate at a pH less than 11.5.
飼主向け情報
- 厳格なスケジュール：処方された通りに正確に投薬してください。投薬を忘れると、けいれん発作の再発や危険な不整脈を引き起こす可能性があります。
- 投薬方法：食事と一緒に与えることで、吸収が良くなり、胃腸の不調を軽減できる場合があります。
- 副作用に注意：ペットが極端に無気力になったり、食欲を失ったり、嘔吐したり、ふらついて歩く（運動失調）場合は、すぐに獣医師に連絡してください。
- 歯の健康：犬の場合、この薬は歯茎の過剰な増殖（歯肉肥厚）を引き起こす可能性があります。定期的な歯科検診と良好な口腔衛生が重要です。
-
安全上の警告：人間用の薬をペットに絶対に与えないでください。動物がこの薬を代謝する方法は人間とは大きく異なり、誤った用量は致命的になる可能性があります。
VetSheet医薬品リファレンスは獣医専門家の臨床意思決定支援を目的としており、専門的判断または製造業者の最新医薬品情報の代替にはなりません。