メトロニダゾール
Metronidazole
2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethanol; Metronidazole benzoate
別名: Flagyl · Metrobactin · Stomorgyl · Metronidazole benzoate · Bayer-5360 · metronidazolum · SC-32642 · NSC-50364 · RP-8823 · SC-10295
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
監査済み v13 用量エビデンス
構造化された用量計算エビデンスAnaerobic infections (extra-label) — less severe anaerobic infections
- q8-12h
NA
必ず考慮する臨床コンテキスト (6)
- 1. Dosages are for metronidazole base unless otherwise noted. If metronidazole benzoate is being used, dosages should be adjusted unless provided by pharmacy as “mg/mL of the base.”
- One mg of metronidazole base = 1.6 mg of metronidazole benzoate. For example, if the calculated metronidazole dose is 200 mg (of the base), the corresponding dose using metronidazole benzoate would be 320 mg (metronidazole benzoate).
- 2. Because of the drug’s extreme bitterness, using a flavored, compounded metronidazole benzoate or putting quartered tablets in an empty gelatin capsule may be considered.
- Anaerobic infections (extra-label):
- ■ Give this medication with food.
- Metronidazole tablets and solution for injection should be stored at temperatures less than 30°C (86°F) and protected from light. The shelf life of divided tablets is 3 days. 5
Hepatic encephalopathy (extra-label)
- q8-12h
NA
必ず考慮する臨床コンテキスト (5)
- 1. Dosages are for metronidazole base unless otherwise noted. If metronidazole benzoate is being used, dosages should be adjusted unless provided by pharmacy as “mg/mL of the base.”
- One mg of metronidazole base = 1.6 mg of metronidazole benzoate. For example, if the calculated metronidazole dose is 200 mg (of the base), the corresponding dose using metronidazole benzoate would be 320 mg (metronidazole benzoate).
- 2. Because of the drug’s extreme bitterness, using a flavored, compounded metronidazole benzoate or putting quartered tablets in an empty gelatin capsule may be considered.
- ■ Give this medication with food.
- Metronidazole tablets and solution for injection should be stored at temperatures less than 30°C (86°F) and protected from light. The shelf life of divided tablets is 3 days. 5
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
メトロニダゾールは、獣医学で広く使用されている合成ニトロイミダゾール系の抗菌および抗原虫薬です。
主な臨床応用：
- 抗原虫作用： ジアルジア、トリコモナス、赤痢アメーバ、大腸バランチジウムに対して高い有効性を示します。
- 抗嫌気性菌作用： 偏性嫌気性菌（バクテロイデス属、クロストリジウム属、フソバクテリウム属など）に対して優れた有効性を示します。敗血症、腹膜炎、深い咬傷などの腸管および全身性の嫌気性菌感染症に頻繁に使用されます。
- 消化管の調節： 炎症性腸疾患（IBD）や慢性大腸炎の管理に頻繁に利用されます。臨床のポイント： 抗菌特性に加えて、メトロニダゾールは細胞性免疫を抑制することにより、消化管において局所的な免疫調節および抗炎症作用を持つと考えられています。
重要な注意事項：
- FDAにより食肉用動物への使用が厳格に禁止されています。
- 非常に苦い金属味があることで知られており、特に猫では深刻な流涎や投薬拒否を引き起こす可能性があります。猫の患者には、唾液に溶けにくく苦味が少ない、調剤された安息香酸メトロニダゾールが好まれることがよくあります。
作用機序
Metronidazole is a concentration-dependent bactericidal agent.
Mechanism Pathway:
- Entry: The drug enters the cell of the organism via passive diffusion.
- Activation: In obligate anaerobes and certain protozoa, the nitro group of metronidazole is reduced by electron-transport proteins (such as ferredoxin).
- DNA Disruption: This reduction process creates highly reactive, short-lived cytotoxic intermediate compounds (free radicals) → These radicals bind to and disrupt DNA and nucleic acid synthesis → leading to rapid cell death.
Because aerobic bacteria lack the specific electron-transport proteins required to reduce and activate the drug, metronidazole is virtually inactive against obligate aerobes.
安全性・警告
禁忌
- Hypersensitivity to metronidazole or other nitroimidazole derivatives
- Food-producing animals (FDA prohibited)
- Pregnancy (potential teratogen, especially in early pregnancy)
- Severe hepatic disease (requires dose reduction)
有害事象
- Neurologic disorders (ataxia, nystagmus, seizures, head-tilt)
- Lethargy and weakness
- Neutropenia
- Hepatotoxicity
- Hematuria
- Anorexia, nausea, vomiting, and diarrhea
- Hypersalivation (especially in cats due to bitter taste)
- CNS toxicity (nystagmus, ataxia, knuckling, head tilt, seizures)
- Haematuria
- Excessive salivation/foaming (especially in cats)
- Impaired sense of smell (reported in explosive detection dogs)
注意事項
Neurotoxicity Warning: In dogs, total daily doses should not exceed 65 mg/kg/day to minimize the risk of central vestibular and cerebellar toxicity. Toxicity can occur with acute high doses or chronic moderate-to-high dose therapy.
- Hepatic Impairment: Use with caution in animals with hepatic dysfunction. Consider reducing the dose to 25-50% of the usual dose in significant liver impairment.
- Pregnancy: Potential teratogen. Avoid use during pregnancy, particularly during the first 3 weeks of gestation, unless benefits clearly outweigh risks.
- Nursing: Consider alternative therapy or switching to milk replacer due to potential tumorigenicity.
- Debilitated Patients: Use with extreme caution in severely debilitated animals.
医薬品相互作用
May induce a disulfiram-like reaction (nausea, vomiting, cramps).
May decrease the metabolism of metronidazole, increasing the likelihood of dose-related side effects.
May increase the metabolism of metronidazole, thereby decreasing blood levels.
May increase the metabolism of metronidazole, thereby decreasing blood levels.
May prolong the prothrombin time (PT). Avoid concurrent use or intensify monitoring.
Should not be used concurrently with Spiramycin (in combination products) as the combination may be antagonistic.
監視
- Clinical efficacy (resolution of diarrhea, negative fecal tests)
- Adverse effects (especially neurologic signs like ataxia or nystagmus)
- Neurological signs (for early detection of CNS toxicity)
- Liver function tests (ALT, AST, Bilirubin) in patients with pre-existing hepatic disease
- Complete Blood Count (for neutropenia)
薬物動態
半減期
吸収
Relatively well absorbed after oral administration. Oral bioavailability in dogs is high (50-100%) and enhanced with food. In cats, bioavailability averages ~65%. In horses, oral bioavailability averages ~80% (decreased by 50% if given rectally).
分布
Highly lipophilic and widely distributed to most body tissues and fluids, including bone, abscesses, the CNS, and seminal fluid. Less than 20% bound to plasma proteins.
代謝
Primarily metabolized in the liver via several pathways, including glucuronidation.
消失
Both metabolites and unchanged drug are eliminated in the urine and feces.
過剰投与
Signs of intoxication in dogs and cats are primarily neurologic and include:
- Anorexia and/or vomiting
- Depression
- Mydriasis (dilated pupils)
- Nystagmus (rapid eye flicking, very common in dogs)
- Ataxia (stumbling, uncoordinated gait)
- Head-tilt, proprioceptive deficits, joint knuckling
- Tremors, seizures, rigidity, and stiffness
These effects may be seen with acute massive overdoses, or more commonly with chronic therapy at doses >60 mg/kg/day (though sometimes seen at 'recommended' doses like 30 mg/kg/day).
Treatment:
- Discontinue metronidazole immediately. Most dogs with mild to moderate signs improve rapidly within 1-2 days of stopping the drug.
- Diazepam has been used successfully to rapidly decrease the CNS effects associated with metronidazole toxicity.
- For acute massive ingestions, standard decontamination protocols (emesis, activated charcoal) apply, but use extreme caution before inducing vomiting if the patient is already showing CNS signs, due to aspiration risk.
製品
製剤
- Oral capsules
- Extended-release oral tablets
- Injection (IV)
- Compounded oral suspensions (often using metronidazole benzoate)
- 5 mg/ml i.v. infusion
- 250 mg tablet
- 500 mg tablet
- 25 mg metronidazole + 46.9 mg spiramycin tablet (Stomorgyl 2)
- 125 mg metronidazole + 234.4 mg spiramycin tablet (Stomorgyl 10)
- 250 mg metronidazole + 469 mg spiramycin tablet (Stomorgyl 20)
- 40 mg/ml oral solution
動物用医薬品
- None (Prohibited for use in food animals by the FDA)
- Metrobactin 250 mg, 500 mg tablets
- Stomorgyl 2, 10, 20 tablets (combination with spiramycin)
ヒト用医薬品
- Metronidazole Oral Tablets: 250 mg & 500 mg (Flagyl, generic)
- Metronidazole Oral Capsules: 375 mg (Flagyl 375, generic)
- Metronidazole Extended-Release Oral Tablets: 750 mg (Flagyl ER)
- Metronidazole Injection: 5 mg/mL in 100 mL vials
- Bismuth Subsalicylate, Metronidazole & Tetracycline HCl Combination (Helidac)
- Flagyl 200 mg, 400 mg tablets
- Metronidazole 5 mg/ml IV infusion
規制ステータス
POM-V classification. Prohibited for use in food-producing animals.
POM-V classification.
規制データなし: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
保管・安定性
Tablets and HCl powder for injection should be stored at temperatures less than 30°C and protected from light. The injection should be protected from light and freezing and stored at room temperature. Compounded metronidazole benzoate suspensions can be stable for up to 90 days or longer depending on the specific vehicle used.
飼主向け情報
飼い主様への重要なご案内：
- 非常に苦い味： メトロニダゾールの錠剤は動物にとって非常に嫌な味がします。絶対に必要でない限り、錠剤を砕かないでください。苦味により、ペット（特に猫）が過剰に唾液を流したり、食事を拒否したりする原因になります。投薬が難しい場合は、味を隠しやすい調剤用の液体（安息香酸メトロニダゾール）について獣医師にご相談ください。
- 食事と一緒に与える： この薬を食事と一緒に与えることで、吸収が良くなり、胃の不快感を軽減することができます。
- 神経症状に注意： ペットに神経毒性の兆候が見られた場合は、直ちに獣医師にご連絡ください。以下の症状に注意してください：ふらつきや「酔っ払った」ような歩き方（運動失調）、眼球の素早い往復運動（眼振）、頭の傾き、極度の無気力、震え、または発作。
- 最後まで飲ませきる： ペットの下痢や症状が改善した場合でも、感染や寄生虫を完全に排除するために、処方された薬は最後まで飲ませきってください。
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。