Inclusion Criteria:
* Adults with spinal cord injury (SCI), 18-75 years of age, at the time of consent
* Have urinary incontinence (UI), with at least 3 leakage episodes/week
* Have urodynamics assessment on file with their provider in past 3 years. If no historical data on file, subject may complete prior to study visit 1.
* Willing and able to comply with study procedures
* Willing and able to provide written informed consent
Exclusion Criteria:
* In phone prescreen, delighted or pleased with current quality of life due to urinary symptoms.
* Indwelling catheter
* History of greater than 4 urinary tract infections per year
* Multiple sclerosis
* Significant heart, liver, kidney, pulmonary, blood, autoimmune or peripheral vascular disease
* Systolic blood pressure \<90 or \>170, diastolic blood pressure \<50 or \>110 after repeated evaluation with proper cuff. This range is the acceptable range stated in the prescribing information for sildenafil (\>90/50 and \<170/110)
* Active cancer
* HIV, Hepatitis B, or Hepatitis C
* Use of systemic nitrates, anabolic steroids, corticosteroids, or long acting PDE5 inhibitors in the past 1 month
* Use of short acting PDE5 inhibitors in the past 1 week
* Use of alpha blockers, anticholinergic agents, bethanechol, or other UI treatment within the past 2 weeks ( 3 weeks for long acting muscarinic receptor antagonists)
* Known allergic reaction to any agent under investigation or required by the protocol
* Females who are pregnant or lactating
* Atonic bladder or high detrusor and high pelvic floor muscle pressure based on previous cystometrogram (CMG) (on file with their provider) that would place subjects at risk for kidney injury
* Any medical condition that, in the opinion of the investigator, would place the subject at increased risk for participation
