Current medical treatment of primary urethral sphincter mechanism incompetence (AKA Spay incontinence/USMI)


Primary sphincter mechanism incompetence (USMI or Spay Incontinence) is a form of incontinence which typically occurs in adult, spayed female dogs and is characterized by incontinence that occurs only when laying down.  Any other timing of incontinence (i.e. while walking) excludes this as a diagnosis. Although a confirmed diagnosis would require urethral pressure profilometry, we will frequently use a positive therapeutic response. In patients that present with a concurrent history of polyuria and polydipsia, it is reasonable to consider overflow incontinence as well.1 The negative impact of urinary continence on a household necessitates quick, effective management. Although not immediately life threatening to the patient, the impact of incontinence on a family can be significant.  Having multiple effective treatment options available has aided veterinarians in the successful management of spay incontinence, ultimately improving the human-animal bond for many families. 


For the treatment of Canine USMI, the most common classes of drugs are estrogens and α-adrenergic agonists for which there is one veterinary FDA approved product in each category.  Incurin® (estriol) is a short acting, naturally occurring estrogen that is indicated for the control of estrogen-responsive urinary incontinence in ovariohysterectomized female dogs. Diethylstilbestrol (DES) is a commonly used synthetic, long acting estrogen, but it is not labeled for use in animals and is only available as a compounded product.2 Proin® (phenylpropanolamine hydrochloride) and Proin ER™ are α-adrenergic agonists and are labeled for control of urinary incontinence due to urethral sphincter hypotonus in dogs. 3,4


As estrogens, Incurin and DES have a trophic effect on the mucosa of the urogenital tract as well as the vasculature and surrounding collagen. Additionally, estrogens increase the production of α-adrenergic receptors in urethral smooth muscle which increases sensitivity to α-agonist activity.5 Efficacy for Incurin in female dogs has been reported as 82% to 92% for improved or complete continence.6 DES has a reported efficacy of approximately 65% complete and 23% improved continence in female dogs.


As a sympathomimetic agent, Proin acts by directly stimulating the α-adrenergic receptors and increasing urethral tone.   It comes in 2 formulations – Proin and Proin ER with ER standing for extended release.  The dose for both products is the same but the ER tablet allows for once daily administration whereas the basic product requires twice daily administration.  Positive response to Proin was seen in 85-90% of dogs.7 Due to its potential use in the manufacture of illicit substances, some states treat phenylpropanolamine as a controlled substance.  Local laws should be checked before dispensing.1


If a patient fails therapy with Proin or Incurin/DES, it is first recommended to try the patient on the alternate medication, as long as no conditions preclude its use. Due to synergistic activity, it is also possible to use the medications in combination.1,8



  1. Acierno MJ and Labato MA, Canine Incontinence, Veterinary Clinics of North America: Small Animal Practice 49, 2, (125-140), (2019).
  2. Incurin [package insert]. Germany: Intervet International B.V, Apr 2016.
  3. Proin [package insert]. Pensacola, FL: Pegasus Laboratories, Inc. Nov 2014.
  4. Proin ER [package insert]. Pensacola, FL: Pegasus Laboratories, Inc. Jan 2019.
  5. Byron JK, Drugs used to manage urinary incontinence in dogs & cats. Plumb’s Therapeutics Brief, (53-58) Dec 2017
  6. Mandigers PJJ and Nell T, Treamtent of bitches with acquired urinary incontinence with oestriol. The Veterinary Record, 149, 764-767 (2001)
  7. Byron JK, Phenylpropanolamine, Plumb’s Therapeutics Brief, (34-35) Sep 2018
  8. Ross S, Urinary incontinence: When good dogs leak.  The Vet Met Team. Course outline. 2013