Medroxyprogesterone Acetate
Medroxyprogesterone acetate (MPA) is a potent, synthetic **progestin** derivative of progesterone. Historically, it has been used in veterinary medicine for behavioral modification (e.g., roaming, inter-male aggression, spraying) and dermatologic conditions (e.g., feline psychogenic alopecia). However, its use has significantly declined due to a **high risk of severe adverse effects**. **Clinical Pearl:** Safer alternatives (such as SSRIs, TCAs, or surgical sterilization) are now strongly preferred for behavioral issues. MPA is still occasionally utilized for specific reproductive indications, such as luteal insufficiency in pregnant bitches, short-term management of benign prostatic hypertrophy, or pituitary dwarfism in German Shepherds. Because of its profound systemic effects, the potential benefits must be carefully weighed against the substantial risks of endocrine and reproductive complications.
Mechanism: MPA mimics endogenous progesterone by binding to intracellular **progesterone receptors**, translocating to the nucleus, and altering gene transcription. It transforms the proliferative endometrium into a secretory endometrium and inhibits spontaneous uterine contractions. * **Endocrine/Reproductive:** Exerts negative feedback on the hypothalamus and pituitary, decreasing **GnRH** โ suppressing **FSH** and **LH** release, thereby inhibiting estrus and spermatogenesis. It also possesses anti-estrogenic properties and decreases plasma testosterone. * **Glucocorticoid Activity:** MPA has significant intrinsic affinity for **glucocorticoid receptors**, leading to profound suppression of the hypothalamic-pituitary-adrenal (HPA) axis (decreased ACTH and cortisol release). * **Metabolic Effects:** Induces peripheral insulin resistance, which can precipitate **diabetes mellitus**. * **Growth Hormone:** In dogs, progestins can stimulate the mammary glands to produce growth hormone, potentially leading to acromegaly.
Dosing by species
- Behavioral disorders (reduce marking in neutered males) ยท 5-20 mg/kg SC or IM three to four times yearly ยท SC/IM ยท 3-4 times yearly ยท Use when all other drugs have been unsuccessful.
- Feline psychogenic alopecia and dermatitis ยท 75-150 mg IM or SC ยท IM/SC ยท Repeat as necessary, but never more often than every 2-3 months
- Progestagen-responsive dermatitis ยท 50-100 mg IM ยท IM ยท May repeat in 3-6 months if needed
- Recurrent abortion secondary to progesterone-deficiency ยท 1-2 mg/kg IM once weekly ยท IM ยท q7d ยท Stop treatment 7-10 days prior to parturition
- Long-term reproductive control ยท 2.5-5 mg PO once weekly; 25 mg injected every 6 months to postpone estrus ยท PO/IM/SC ยท q7d (PO) or q6mo (Injection)
- Long-term reproductive control ยท 2 mg/kg IM q5 months ยท IM ยท q5 months
- Management of spring transition, prevention of estrus behavior, induction of estrous cycle synchrony, pregnancy maintenance, modification of stallion behavior ยท 500 to 800 mg IM ยท IM ยท Interval varies ยท Most injections last 2 to 3 months ยท Will not prevent pregnancy loss and does not stop cyclicity.
- Antipruritic and to suppress ovulation ยท 0.025-1 mL (3 mg/100 grams body weight) IM once every 4-6 weeks ยท IM ยท q4-6w ยท May cause obesity, fatty liver, polydipsia/polyuria and lethargy if used repeatedly.
Routes of administration
Contraindications
- Pre-pubescent cats or dogs
- Diabetes mellitus
- Pseudopregnant bitches
- Females in diestrus or with prolonged heat
- Uterine hemorrhage or discharge
- Pregnancy (unless specifically treating hypoluteoidism)
Adverse effects
- Increased thirst (polydipsia) and urination (polyuria)
- Increased appetite (polyphagia) and weight gain
- Depression, lethargy, and personality changes
- Adrenocortical depression (HPA axis suppression)
- Mammary changes (enlargement, milk production, neoplasia)
- Diabetes mellitus
- Pyometra / Cystic endometrial hyperplasia
- Temporary inhibition of spermatogenesis
- Permanent local alopecia, skin atrophy, and depigmentation (with SC injection)
- Acromegaly (in dogs)
Drug interactions
- Aminoglutethimide ยท May decrease medroxyprogesterone effects
- Felbamate ยท May increase medroxyprogesterone metabolism
- Rifampin ยท May decrease progestin activity due to microsomal enzyme induction and increased progestin metabolism
Monitoring
- Weight
- Blood glucose (draw baseline before therapy and monitor periodically)
- Mammary gland development
- Adrenocortical function
- Clinical efficacy
Overdose
No specific reports on inadvertent acute overdosage were located. Overdose is expected to manifest as an exaggeration of adverse effects, including profound adrenal suppression, lethargy, metabolic derangements (hyperglycemia), and reproductive tract changes. Treatment should be supportive and symptomatic.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.