Estriol
Estriol is a synthetic, short-acting **estrogen** with a high affinity for estrogen receptors in the lower urogenital tract. It is primarily used in veterinary medicine for the management of **urethral sphincter mechanism incompetence (USMI)** in spayed female dogs. By increasing resting urethral muscle tone, it improves urodynamic function and helps resolve urinary incontinence. > **Clinical Warning:** Estriol should not be used in intact bitches or animals under 1 year of age. Long-term use carries a low risk of bone marrow suppression.
Mechanism: Estriol binds to **estrogen receptors** in the lower urogenital tract → increases resting urethral muscle tone and mucosal vascularity → improves urodynamic function and urethral closure pressure.
Dosing by species
- All indications · Do not use. · PO · N/A · N/A · Contraindicated or not recommended for use in cats.
- Urethral sphincter mechanism incompetence (USMI) in spayed bitches · Starting dose: 1 mg/dog. If successful, reduce to 0.5 mg/dog. If unsuccessful, increase to 2 mg/dog. Minimum effective dose: 0.5 mg/dog. Maximum dose: 2 mg/dog. · PO · q24h (can consider q48h for maintenance once response is seen) · Long-term · Alternate-day dosing can be considered once a response has been seen.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Intact bitches
- Polyuria/polydipsia (PU/PD) of unknown origin
- Animals < 1 year old
Adverse effects
- Oestrogenic effects (swollen vulva, attractiveness to males) in 5-9% of dogs at 2 mg/day
- Bone marrow suppression (rare, but possible with long-term use)
- Gastrointestinal upset (mild)
Drug interactions
- Phenylpropanolamine · Concurrent administration may potentiate efficacy in treating urinary incontinence (synergistic effect). · minor
Monitoring
- Clinical response (resolution of urinary incontinence)
- Complete Blood Count (CBC) periodically for long-term use to monitor for bone marrow suppression
- Signs of estrogenic side effects (vulvar swelling)
Overdose
Overdose may lead to exaggerated estrogenic effects, including pronounced vulvar swelling, attractiveness to male dogs, and potentially an increased risk of **bone marrow suppression** (anemia, thrombocytopenia, leukopenia). Treatment is supportive and symptomatic; discontinue the drug until signs resolve.
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.