酢酸メゲストロール

Megestrol Acetate

17-alpha-acetoxy-6-dehydro-6-methylprogesterone

別名: Ovaban · Megace · Ovarid · BDH-1298 · compound 5071 · megestroli acetas · NSC-71423 · SC-10363 · Acestrol · Borea · Endace · Gynodal · Maygace · Megastrol · Megefren · Megestat · Megestil · Megestin · Megostat · Meltonar · Meprogest · Mestrel · Niagestine · Prazoken · Varigestrol

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

2.2 mg/kg once daily for 8 days starting during the first 3 days of proestrus. May be prolonged with 2.2 mg/kg/day for 4 days, then 0.55 mg/kg/day for 16-20 days.PO· q24h· 8-24 days

体重— 🐕

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
To halt cycle in proestrus	2.2 mg/kg once daily for 8 days starting during the first 3 days of proestrus. May be prolonged with 2.2 mg/kg/day for 4 days, then 0.55 mg/kg/day for 16-20 days.	PO	q24h	8-24 days	🌎 NA
To postpone an anticipated cycle	0.55 mg/kg/day for 32 days, beginning at least 7 days prior to proestrus	PO	q24h	32 days	🌎 NA
To delay an anticipated heat during anestrus	0.55 mg/kg PO for 32 days initiated 7 days prior to proestrus.	PO	q24h	32 days	🌎 NA
Estrus suppression (general)	2 mg/kg PO once daily for 8 consecutive days	PO	q24h	8 days	🌎 NA
Temporary estrus postponement (general)	0.5 mg/kg PO once daily in late anestrus	PO	q24h	Variable	🌎 NA
Benign prostatic hypertrophy	0.5 mg/kg PO daily for 4-8 weeks	PO	q24h	4-8 weeks	🌎 NA
Benign prostatic hypertrophy	0.55 mg/kg PO daily	PO	q24h	Variable	🌎 NA
Benign prostatic hypertrophy	0.1-0.5 mg/kg per day for 3-8 weeks	PO	q24h	3-8 weeks	🌎 NA
Pseudocyesis (false pregnancy)	0.5 mg/kg PO once daily for 8 days	PO	q24h	8 days	🌎 NA
To prevent vaginal hyperplasia development	2.2 mg/kg PO for 7 days early in proestrus	PO	q24h	7 days	🌎 NA
Treatment of severe galactorrhea	0.55 mg/kg PO once daily for 7 days	PO	q24h	7 days	🌎 NA
Adjunctive treatment of aggressive or unacceptable masculine behavior	1.1-2.2 mg/kg PO once daily for 2 weeks, then 0.5-1.1 mg/kg once daily for 2 weeks.	PO	q24h	4 weeks	🌎 NA

To halt cycle in proestrus: Bitch must be controlled until behavioral signs of estrus disappear.
To delay an anticipated heat during anestrus: Recommend doing vaginal cytology prior to therapy. Do not repeat therapy more often than once every 6 months.
Benign prostatic hypertrophy: Best used to maintain breeding potential for short time prior to castration; use with caution.
Adjunctive treatment of aggressive or unacceptable masculine behavior: Should be used with behavior modification.

猫

適応	用量	経路	頻度	期間	地域
Suppression of estrus (in anestrus)	5 mg/cat PO every 2 weeks or 2.5 mg/cat per week (better if divided into 2 doses given every 3.5 days)	PO	q7-14d	Variable	🌎 NA
Suppression of estrus (in proestrus)	5 mg/cat per day for 4 days, then 5 mg PO every 2 weeks.	PO	q24h then q14d	Variable	🌎 NA
Suppression of estrus (behavioral estrus)	5 mg/day PO until estrus stops (generally within 3-5 days), then 2.5-5 mg PO once weekly for 10 weeks	PO	q24h then q7d	10+ weeks	🌎 NA
Postponement of estrus (started during diestrus)	2.5 mg PO daily for 8 weeks	PO	q24h	8 weeks	🌎 NA
Postponement of estrus (started during anestrus)	2.5 mg PO once weekly for up to 18 months.	PO	q7d	Up to 18 months	🌎 NA
Prevention of estrus	5 mg daily PO for 3 days as soon as behavioral signs of estrus are seen	PO	q24h	3 days	🌎 NA
Idiopathic feline miliary dermatitis	2.5-5 mg once every other day, followed by weekly maintenance dosages.	PO	q48h then q7d	Variable	🌎 NA
Appetite stimulant	0.25-0.5 mg/kg q24h for 3-5 days, then q48-72h	PO	q24h then q48-72h	Variable	🌎 NA
Alternative treatment for immune-mediated skin diseases	2.5-5 mg PO once daily for 10 days, then every other day	PO	q24h then q48h	Variable	🌎 NA
Adjunctive therapy of eosinophilic granulomas	0.5 mg/kg PO once daily for 2 weeks, then twice weekly as needed	PO	q24h then twice weekly	Variable	🌎 NA
Eosinophilic ulcers	5-10 mg PO every other day for 10-14 doses, then every 2 weeks as needed	PO	q48h then q14d	Variable	🌎 NA
Feline atopy ('last ditch' alternative)	2.5-5 mg per cat PO every 48 hours for 1-3 weeks. This dose is then used once weekly.	PO	q48h then q7d	Variable	🌎 NA
Feline plasma cell gingivitis	2.5 mg PO once daily for 10 days, then once every other day for 5 treatments, then as needed	PO	q24h then q48h	Variable	🌎 NA
Feline endocrine alopecia (FEA)	5 mg PO every second to third day initially, then 2.5 mg PO once to twice weekly	PO	q48-72h then 1-2x weekly	Variable	🌎 NA
Feline psychogenic alopecia and dermatitis	2.5-5 mg every other day initially, then taper to the lowest maintenance dosage possible, given weekly as needed	PO	q48h then q7d	Variable	🌎 NA
Persistent hematuria and urethritis in a non-obstructed cat	2.5-5 mg PO once daily to every other day (with prednisone: 2.5-5 mg PO daily)	PO	q24-48h	Variable	🌎 NA
Urine marking, intraspecies aggression, anxiety	2 mg/kg/day for 5 days, then 1 mg/kg/day for 5 days, then 0.5 mg/kg/day for 5 days	PO	q24h	15 days	🌎 NA
To reduce marking in neutered male cats	2.5-10 mg (total dose) per cat PO once daily for one week, then reduce to once or twice weekly.	PO	q24h then 1-2x weekly	Variable	🌎 NA
Urine marking, intraspecies aggression, anxiety	5 mg per cat once daily for 2 weeks, then wean to lowest effective maintenance dose.	PO	q24h	Variable	🌎 NA

Postponement of estrus (started during anestrus): Recommend allowing cat to have a cycle (unmedicated) before beginning another treatment cycle.
Prevention of estrus: Next estrus period will occur in approximately 4 weeks.
Idiopathic feline miliary dermatitis: Reserve use for severe cases; explain risks to owner and do not exceed 2.5 mg per week during maintenance phase.
Eosinophilic ulcers: Alone or in combination with methylprednisolone acetate.
Feline endocrine alopecia (FEA): Secondary therapy (thyroid hormone replacement first choice).
Persistent hematuria and urethritis in a non-obstructed cat: Adjunctive therapy.
To reduce marking in neutered male cats: When all other drugs have been unsuccessful.
Urine marking, intraspecies aggression, anxiety: Has poor efficacy in neutered males (50%) and spayed females (10%); many potential adverse effects.

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。

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概要

酢酸メゲストロールは、強力な合成プロゲスチンであり、顕著な抗エストロゲン作用および内因性グルココルチコイド特性を持っています。

歴史的に、特に猫の皮膚疾患や行動問題に広く使用されてきましたが、重篤で不可逆的な副作用（重度の副腎皮質抑制や医原性糖尿病など）のリスクが高いため、現在では猫への使用は大幅に減少しています。

犬においては、メスの発情延期および偽妊娠の軽減としてFDAに承認されており、オスでは良性前立腺肥大（BPH）に適応外使用されます。人間医学では、食欲増進剤や進行性乳がん・子宮内膜がんの緩和ケアとして利用されています。

作用機序

Megestrol acetate exerts its effects through multiple receptor pathways:

Progesterone Receptors: Binds to progestin receptors → provides negative feedback to the hypothalamus and pituitary → inhibits the release of GnRH, LH, and FSH → suppresses estrus and ovulation.
Glucocorticoid Receptors: Possesses significant intrinsic glucocorticoid activity → binds to glucocorticoid receptors → causes profound suppression of the hypothalamic-pituitary-adrenal (HPA) axis and induces insulin resistance.
Anti-estrogenic Activity: Modifies target tissue response to estrogens.

Clinical Pearl: Unlike some other progestins, megestrol acetate does not possess significant anabolic or masculinizing effects on the developing fetus.

安全性・警告

禁忌

Uterine disease (e.g., pyometra, endometritis)
Diabetes mellitus
Mammary neoplasias
Prior to the first estrous cycle in dogs
Anestrus therapy in dogs with abnormal cycles
During diestrus or in the presence of uterine hemorrhage
Treatment of pseudopregnancy in bitches (per manufacturer, though off-label protocols exist)

有害事象

Profound adrenocortical suppression (especially in cats)
Transient or permanent diabetes mellitus
Cystic endometrial hyperplasia (CEH) and pyometra
Mammary hypertrophy and neoplasia
Increased appetite and weight gain
Polydipsia and polyuria (PU/PD)
Lethargy and personality changes
Hepatotoxicity (rare)
Acromegaly (dogs)
Lactation (rare)

注意事項

Black Box Warning Equivalent: Megestrol acetate can induce profound, life-threatening adrenocortical suppression and iatrogenic Addisonian crisis, particularly in cats.

Adrenal Suppression: Because of its glucocorticoid activity, patients may require exogenous stress-dose steroids during periods of trauma, surgery, or severe illness.
Reproductive Risks: High risk of inducing cystic endometrial hyperplasia (CEH) and pyometra. A thorough reproductive history, physical exam, mammary palpation, and vaginal cytology are strongly recommended before initiating therapy for reproductive control.
Infection Risk: May exacerbate latent viral infections (e.g., Feline Herpesvirus-1).
Human Safety: Pregnant women should handle this medication with extreme caution (FDA Category X in early pregnancy).

医薬品相互作用

Corticosteroids

Concurrent long-term use may exacerbate adrenocortical suppression and increase the risk of diabetes mellitus.

Rifampin

May decrease progestin activity due to microsomal enzyme induction, resulting in increased progestin metabolism.

監視

Body weight
Blood glucose (draw baseline before therapy)
Mammary gland development and appearance (nodules/hypertrophy)
Adrenocortical function (ACTH stimulation test if indicated)
Liver enzymes (especially with long-term treatment)
Clinical efficacy

薬物動態

半減期

犬8 days

吸収

Well absorbed from the gastrointestinal tract.

分布

Widely distributed. Detectable amounts of progestins enter the milk of nursing mothers.

代謝

Appears to be metabolized completely in the liver to conjugates and free steroids.

消失

Excreted primarily via urine and feces.

過剰投与

Acute toxicity from overdosage has not been reported. In humans, massive doses (up to 800 mg/day) caused no observable acute adverse reactions.

Chronic Toxicity in Dogs:

Dosages of 0.1-0.25 mg/kg/day PO for 36 months yielded no gross abnormalities, but histologically, cystic endometrial hyperplasia (CEH) was noted (resolved when therapy was discontinued).
Dosages of 0.5 mg/kg/day PO for 5 months caused reversible uterine hyperplasia.
Dosages of 2 mg/kg/day demonstrated early cystic endometritis within 64 days.

製品

製剤

Oral tablets
Oral suspension

動物用医薬品

Megestrol Acetate Oral Tablets: 5 mg, 20 mg (Ovaban)

ヒト用医薬品

Megestrol Acetate Tablets: 20 mg, 40 mg (Megace)
Megestrol Acetate Suspension: 40 mg/mL, 125 mg/mL (Megace, Megace ES)

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 tablets in well-closed containers at a temperature of less than 40°C. The veterinary manufacturer recommends storing the tablets from 2°-30°C (36°-86°F). Tablets may be crushed and administered with food.

飼主向け情報

酢酸メゲストロールは強力なホルモン薬です。効果がある一方で、特に猫においては重大なリスクを伴います。

リスクの理解：この薬は、糖尿病、重篤な子宮感染症（子宮蓄膿症）、副腎の抑制など、深刻な副作用を引き起こす可能性があります。
注意深い観察：ペットの飲水量と尿量を注意深く観察してください。異常に水を飲んだり尿をしたりし始めた場合は、糖尿病の兆候である可能性があるため、すぐに獣医師に連絡してください。
身体の変化：ペットの乳腺（お腹のあたり）に腫れ、しこり、分泌物がないか定期的に確認してください。変化があれば獣医師に報告してください。
行動と活力：極度の無気力、脱力感、または行動の突然の変化に注意してください。
急に服用を中止しない：ペットがこの薬を長期服用している場合、体が適応する時間が必要なため、獣医師に相談せずに急に服用を中止しないでください。
人間の安全性：妊娠中の女性はこの薬の取り扱いを避けてください。

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。