フェノバルビタール
Phenobarbital
Phenobarbital sodium, phenylethylbarbituric acid, phenylethylmalonylurea
別名: Luminal Sodium · Solfoton · Epiphen · Epityl · Phenoleptil · Fenobarbital · Phenemalum · Phenobarbitalum · Phenobarbitone · Phenylethylbarbituric acid · Phenylethylmalonylurea · Phenobarbital sodium
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 |
|---|---|---|---|---|---|
| Idiopathic epilepsy (initial dose) | 2-2.5 mg/kg | PO | q12h | — | 🌎 NA |
| Idiopathic epilepsy (initial maintenance) | 2.5-3 mg/kg | PO | q12h | — | 🌎 NA |
| Idiopathic epilepsy (rapid loading) | 16-20 mg/kg once IV loading dose, then 2-5 mg/kg PO q12h | IV/PO | Once then q12h | — | 🌎 NA |
| Status epilepticus | 2-5 mg/kg bolus (repeat at 20 min intervals up to 2x), or add to diazepam infusion at 2-10 mg/hour | IV | PRN | — | 🌎 NA |
| Status epilepticus (post-benzodiazepine) | 5-8 mg/kg | IV | q4-6h | — | 🌎 NA |
| Sialadenosis | 1 mg/kg | PO | q12h | 3 months | 🌎 NA |
| Sedation | 2.2-6.6 mg/kg | PO | q12h | — | 🌎 NA |
| Irritable bowel syndrome | 2.2 mg/kg | PO | q12h | — | 🌎 NA |
| Compulsive behaviors (adjunctive) | 2-20 mg/kg | PO | q12-24h | — | 🌎 NA |
- Idiopathic epilepsy (initial dose): Monitor serum concentrations 2-3 weeks after initiating. Target 20-35 mcg/mL.
- Idiopathic epilepsy (initial maintenance): Perform baseline CBC, chem, UA before starting.
- Status epilepticus: If seizures persist after diazepam therapy.
- Status epilepticus (post-benzodiazepine): Continue until seizures are under control.
- Sialadenosis: Slowly weaned off after 3 months.
猫
| 適応 | 用量 | 経路 | 頻度 | 期間 | 地域 |
|---|---|---|---|---|---|
| Idiopathic epilepsy (initial dose) | 2-2.5 mg/kg | PO | q12h | — | 🌎 NA |
| Idiopathic epilepsy (maintenance) | 1-2 mg/kg | PO | q12h | — | 🌎 NA |
| Idiopathic epilepsy (rapid loading) | 16-20 mg/kg once IV loading dose, then 1-5 mg/kg PO q12h | IV/PO | Once then q12h | — | 🌎 NA |
| Status epilepticus | 2-5 mg/kg bolus (repeat at 20 min intervals up to 2x), or add to diazepam infusion at 2-10 mg/hour | IV | PRN | — | 🌎 NA |
| Emergency seizure control | 3 mg/kg (repeat q20m up to 24 mg/kg/24h) OR 10 mg/kg bolus | IV | PRN | — | 🌎 NA |
| Sedation (situational distress/travel) | 2-3 mg/kg | PO | PRN | — | 🌎 NA |
- Idiopathic epilepsy (initial dose): Optimum therapeutic levels 23-30 mcg/mL.
- Idiopathic epilepsy (maintenance): Adjust based on serum levels.
- Status epilepticus: If seizures persist after diazepam therapy.
- Emergency seizure control: Given along with IV diazepam.
- Sedation (situational distress/travel): For controlling excessive vocalization.
フェレット
| 適応 | 用量 | 経路 | 頻度 | 期間 | 地域 |
|---|---|---|---|---|---|
| Seizures | 1-2 mg/kg | PO | q8-12h (2-3 times daily) | — | 🌎 NA |
| Seizures (rapid loading) | Loading dose of 16-20 mg/kg once IV; maintenance dose of 1-2 mg/kg PO q8-12h | IV/PO | q8-12h | — | 🌎 NA |
馬
| 適応 | 用量 | 経路 | 頻度 | 期間 | 地域 |
|---|---|---|---|---|---|
| Seizures (Adults) | Loading dose of 12 mg/kg IV over 20 minutes, then 6.65 mg/kg IV over 20 minutes q12h | IV | q12h | — | 🌎 NA |
| Seizures (Adults) | Loading dose of 16-20 mg/kg once IV; maintenance dose of 1-5 mg/kg PO twice daily | IV/PO | q12h | — | 🌎 NA |
| Seizures (Foals) | Loading dose of 16-20 mg/kg once IV; maintenance dose of 100-500 mg (total dose) PO twice daily | IV/PO | q12h | — | 🌎 NA |
| Seizures (Foals) | 20 mg/kg diluted and infused over 25-30 minutes IV, then 9 mg/kg infused as above q8h | IV | q8h | — | 🌎 NA |
牛
| 適応 | 用量 | 経路 | 頻度 | 期間 | 地域 |
|---|---|---|---|---|---|
| Enzyme induction in organochlorine toxicity | 5 grams | PO | Daily | 3-4 weeks on, 3-4 weeks off, repeat | 🌎 NA |
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
フェノバルビタールは長時間作用型のバルビツール酸系薬であり、獣医学におけるてんかん発作管理の基礎となる薬剤です。
- 主な用途: 猫の特発性てんかん治療の第一選択薬として広く認識されており、犬や馬においても最も一般的で効果的な第一選択または補助的な抗てんかん薬の一つです。
- 臨床的利点: 良好な薬物動態プロファイル、相対的な安全性、証明された有効性、および低コストを提供します。催眠量以下の用量でてんかんを効果的にコントロールできます。
- 肝臓の自己誘導: フェノバルビタールのユニークな薬理学的特徴は、肝臓のシトクロムP450酵素を誘導する能力です。時間の経過とともに、患者の肝臓は薬物をより効率的に代謝するようになり、治療血清レベルを維持するために用量の増加が必要になることがよくあります。
- その他の用途: 発作のコントロール以外にも、経口鎮静剤（猫の旅行不安や過剰な鳴き声など）として使用されることがあり、ベンゾジアゼピン系薬による初期コントロール後のてんかん重積状態の管理にも使用されます。
作用機序
Phenobarbital exerts its antiseizure and sedative effects through multiple mechanisms within the central nervous system:
- GABA-A Receptor Modulation: Phenobarbital binds to the allosteric barbiturate site on the GABA-A receptor. Unlike benzodiazepines (which increase the frequency of chloride channel opening), phenobarbital increases the duration of chloride channel opening → enhanced influx of chloride ions → hyperpolarization of the postsynaptic neuronal membrane → increased seizure threshold and decreased spread of seizure activity.
- Glutamate Inhibition: It inhibits the release of excitatory neurotransmitters, specifically glutamate, thereby dampening CNS excitability.
- Calcium Channel Blockade: At high (anesthetic) doses, it inhibits the uptake of calcium at nerve endings, further reducing neurotransmitter release.
- Other Neurotransmitters: It has also been shown to inhibit the release of acetylcholine and norepinephrine.
安全性・警告
禁忌
- Known hypersensitivity to barbiturates
- Severe liver disease
- Nephritis (large doses)
- Severe respiratory depression
- Hepatic dysfunction
- Severe renal impairment
有害事象
- Dogs: Transient anxiety, agitation, or lethargy upon initiation
- Dogs: Polydipsia (PD), polyuria (PU), and polyphagia (PP)
- Dogs: Sedation and ataxia (especially at higher serum levels)
- Dogs: Elevated liver enzymes (ALT, ALP) - common and not always indicative of failure
- Dogs: Hepatotoxicity (uncommon, usually at levels >30-40 mcg/mL)
- Dogs: Rare blood dyscrasias (anemia, thrombocytopenia, neutropenia)
- Dogs: Rare superficial necrolytic dermatitis (SND)
- Cats: Ataxia, persistent sedation, lethargy
- Cats: Polyphagia, weight gain, PU/PD
- Cats: Rare immune-mediated reactions and bone marrow hypoplasia
- Cats: Coagulopathies (at very high doses)
- Blood dyscrasias (rare)
注意事項
Intravenous Administration: Must be given SLOWLY (not more than 60 mg/minute). Too rapid IV administration may cause profound respiratory depression.
Tissue Irritation: Do NOT administer subcutaneously (SC) or perivascularly. The injectable solution is highly alkaline and very irritating, potentially causing significant tissue necrosis.
- Patient Conditions: Use with extreme caution in patients that are hypovolemic, anemic, have borderline hypoadrenal function, or have pre-existing cardiac or respiratory disease.
- Feline Sensitivity: Cats are particularly sensitive to the respiratory depressant effects of barbiturates.
- Hepatic Function: Chronic administration induces hepatic microsomal enzymes. Monitor liver function closely, especially in dogs, as hepatotoxicity can occur at high serum concentrations.
- Endocrine Testing: Can alter thyroid testing (decreased T4, normal T3, normal/increased TSH) and may cause a false positive low-dose dexamethasone suppression test.
医薬品相互作用
Increased risk for hepatotoxicity, particularly with large or chronic doses of barbiturates.
Increased risk for hepatotoxicity secondary to carprofen metabolites.
May prolong phenobarbital effects.
Barbiturates may affect phenytoin metabolism, and phenytoin may alter barbiturate levels.
May induce enzymes that increase the metabolism of barbiturates.
Phenobarbital reduces levetiracetam elimination half-life by about 50% in dogs.
Phenobarbital may decrease anticoagulant effects by lowering serum concentrations.
Phenobarbital increases the metabolism and clearance of corticosteroids, potentially reducing their efficacy.
Phenobarbital decreases doxycycline serum concentrations; effect may persist for weeks after discontinuation.
Phenobarbital increases theophylline metabolism, lowering its serum concentrations.
May increase the effects of phenobarbital while phenobarbital may decrease chloramphenicol levels.
Increased hepatic metabolism and clearance of T4
Inhibits phenobarbital metabolism
監視
- Anticonvulsant efficacy (seizure frequency and severity)
- Adverse effects (CNS depression, ataxia, PU/PD, weight gain)
- Serum phenobarbital levels (Dogs: 20-35 mcg/mL; Cats: 23-30 mcg/mL). Wait 5-6 half-lives (12-14 days in dogs, 9-10 days in cats) before measuring steady-state levels.
- Routine CBC, liver enzymes (especially ALT and AST), and bilirubin at least every 6 months during chronic therapy.
- Serum phenobarbital concentrations (trough levels)
- Liver enzymes (ALT, ALP, GGT)
- Serum bile acids
- Complete Blood Count (CBC)
- Renal function
薬物動態
半減期
吸収
Slowly absorbed from the GI tract. Bioavailability is approximately 90% in dogs and practically complete in adult horses. Peak levels occur 4-8 hours after oral dosing in dogs.
分布
Widely distributed throughout the body. Due to lower lipid solubility compared to other barbiturates, it does not distribute as rapidly into the CNS. Protein binding is 40-60%. Volume of distribution: Dogs ~0.75 L/kg, Horses ~0.8 L/kg.
代謝
Metabolized in the liver primarily by hydroxylated oxidation to p-hydroxyphenobarbital; sulfate and glucuronide conjugates are also formed. Induces hepatic microsomal enzymes, leading to auto-induction and increased clearance over time.
消失
Approximately 25% is excreted unchanged by the kidneys. Alkalinizing the urine or increasing urine flow increases excretion rates.
過剰投与
Clinical Signs of Toxicity:
- Dogs: Ataxia, lethargy, sedation, recumbency, depression, hypothermia, and coma.
- Cats: Ataxia, sedation, and recumbency.
Treatment:
- Decontamination: Removal of ingested product from the gut if recent. Activated charcoal is highly effective and acts as a 'sink' to draw the drug from the vasculature back into the gut, enhancing clearance even if the drug was given parenterally.
- Supportive Care: Provide intensive respiratory and cardiovascular support.
- Enhanced Elimination: Forced alkaline diuresis can substantially augment elimination in patients with normal renal function. Peritoneal dialysis or hemodialysis may be helpful in severe intoxications or anuric patients.
製品
製剤
- Tablets
- Capsules
- Oral elixir/suspension
- 12.5 mg oral tablet
- 15 mg oral tablet
- 25 mg oral tablet
- 30 mg oral tablet
- 50 mg oral tablet
- 60 mg oral tablet
- 100 mg oral tablet
- 4% (40 mg/ml) oral solution
- 15 mg/5 ml oral solution
- 15 mg/ml injectable solution
- 30 mg/ml injectable solution
- 60 mg/ml injectable solution
- 200 mg/ml injectable solution
動物用医薬品
- None currently labeled for veterinary use.
- Epiphen
- Epityl
- Phenoleptil
ヒト用医薬品
- Phenobarbital Tablets: 15 mg, 16 mg, 30 mg, 60 mg, 90 mg, & 100 mg (Solfoton, generic)
- Phenobarbital Elixir: 15 mg/5mL, 20 mg/5mL (generic)
- Phenobarbital Sodium Injection: 30 mg/mL, 60 mg/mL, 65 mg/mL, & 130 mg/mL (Luminal Sodium, generic)
- Phenobarbital oral solution
規制ステータス
POM-V, POM CD SCHEDULE 3
規制データなし: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
保管・安定性
Tablets: Store in tight, light-resistant containers at room temperature (15-30°C); protect from moisture. Elixir: Store in tight containers at 20-25°C. Injection: Store at room temperature (15-30°C). Do not add to acidic solutions or use if precipitate forms.
飼主向け情報
飼い主様への重要なご案内：
- 一貫性が鍵です: てんかん治療を成功させるためには、毎日同じ時間にこの薬を投与する必要があります。1回でも投与を忘れると、発作を引き起こす可能性があります。
- 初期の副作用: 薬の服用を始めたばかりの頃、ペットは「酔っ払った船乗り」のように振る舞うことがあります（ふらつき、眠気、または異常な空腹感/喉の渇き）。これは正常な反応であり、体が慣れるにつれて通常1〜2週間で大幅に改善します。
- 急に中止しないでください: 獣医師に相談することなく、突然この薬の投与を中止しないでください。重度で生命を脅かす発作を引き起こす可能性があります。
- モニタリング: 獣医師は、血中の薬物濃度を調べ、肝機能を監視するために定期的に採血を行う必要があります。これはペットの安全を守り、用量が適切であることを確認するために非常に重要です。
- 安全な保管: この薬は子供や他のペットの手の届かない場所に厳重に保管してください。子供が開けにくい容器に入れて保管してください。
- 獣医師に連絡するタイミング: ペットの発作が抑えられない場合、極度に無気力になった場合、または目や歯茎の黄色み（黄疸）や重度の脱力感などの兆候に気づいた場合は、すぐに獣医師に連絡してください。
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。