Megestrol Acetate
17-alpha-acetoxy-6-dehydro-6-methylprogesterone
Also known as: 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
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| 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.
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| 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.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Megestrol acetate is a potent, synthetic progestin with significant anti-estrogenic and intrinsic glucocorticoid properties.
Historically, it was widely used in veterinary medicine—particularly in cats—for a variety of dermatological and behavioral conditions. However, its use in felines has drastically declined in modern practice due to the high risk of severe, sometimes irreversible adverse effects, including profound adrenocortical suppression and iatrogenic diabetes mellitus.
In dogs, it remains FDA-approved for the postponement of estrus and alleviation of false pregnancy in females, and is used off-label for benign prostatic hypertrophy (BPH) in males. In human medicine, it is utilized as an appetite stimulant and for the palliative treatment of advanced breast or endometrial carcinomas.
Mechanism of action
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.
Safety & warnings
Contraindications
- 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)
Adverse effects
- 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)
Precautions
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).
Drug interactions
Concurrent long-term use may exacerbate adrenocortical suppression and increase the risk of diabetes mellitus.
May decrease progestin activity due to microsomal enzyme induction, resulting in increased progestin metabolism.
Monitoring
- 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
Pharmacokinetics
Half-life
Absorption
Well absorbed from the gastrointestinal tract.
Distribution
Widely distributed. Detectable amounts of progestins enter the milk of nursing mothers.
Metabolism
Appears to be metabolized completely in the liver to conjugates and free steroids.
Elimination
Excreted primarily via urine and feces.
Overdose
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.
Available products
Formulations
- Oral tablets
- Oral suspension
Veterinary
- Megestrol Acetate Oral Tablets: 5 mg, 20 mg (Ovaban)
Human-labeled
- Megestrol Acetate Tablets: 20 mg, 40 mg (Megace)
- Megestrol Acetate Suspension: 40 mg/mL, 125 mg/mL (Megace, Megace ES)
Regulatory status
No regulatory data for: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
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.
Client information
Megestrol acetate is a powerful hormone medication. While it can be effective, it carries significant risks, especially in cats.
- Understand the Risks: This drug can cause serious side effects, including diabetes, severe uterine infections (pyometra), and suppression of the adrenal glands.
- Monitor Closely: Watch your pet's water intake and urination closely. If they start drinking or urinating excessively, contact your veterinarian immediately, as this could be a sign of diabetes.
- Physical Changes: Check your pet's mammary glands (belly area) regularly for any swelling, lumps, or discharge. Report any changes to your vet.
- Behavior and Energy: Note any extreme lethargy, weakness, or sudden changes in behavior.
- Do Not Stop Abruptly: If your pet has been on this medication long-term, do not stop it suddenly without consulting your veterinarian, as their body may need time to adjust.
- Human Safety: Pregnant women should avoid handling this medication.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.