From the Guidelines

Solifenacin (VESIcare) and Darifenacin (Enablex) are both effective for managing urgency UI, but they have different adverse effect profiles.

Both Solifenacin and Darifenacin are antimuscarinics used to treat urgency UI, with equally efficacious results in managing urgency UI and achieving continence rates 1.
Adverse effects: The most commonly reported adverse effects associated with antimuscarinics, including Solifenacin and Darifenacin, are dry mouth, constipation, and blurred vision 1.
Discontinuation due to adverse effects: Darifenacin has a lower risk of discontinuation due to adverse effects compared to Solifenacin, with no statistically significant difference in treatment discontinuation rates between Darifenacin and placebo 1.
Specific adverse effects:
- Solifenacin: associated with dry mouth and constipation more commonly than tolterodine, but blurred vision was more common with tolterodine than Solifenacin 1.
- Darifenacin: has a similar risk for discontinuation due to adverse effects as placebo, and no significant difference in adverse effects compared to tolterodine 1.

When choosing between Solifenacin and Darifenacin, clinicians should consider tolerability, adverse effect profile, ease of use, and cost of medication 1.
Patient characteristics, such as age, race, comorbid conditions, or baseline UI, do not affect the outcomes of the various pharmacologic medications, but adherence to pharmacologic treatments for UI is poor 1.

The FDA Drug Labels for solifenacin (PO), darifenacin (PO) do not address this question.

From the Research

Solifenacin and darifenacin are both selective muscarinic receptor antagonists used to treat overactive bladder (OAB) symptoms 2.
A study comparing the two drugs found that both treatment groups showed a reduction in all OAB symptoms, but with no notable difference between the two groups 2.
However, solifenacin showed statistically greater improvements in quality of life, better overall treatment satisfaction, and a decreased incidence of dry mouth after 3 months of treatment compared to the darifenacin group 2.

Solifenacin has been shown to be effective in decreasing urgency episodes, incontinence, urge incontinence, and pad usage, and increasing the volume voided per micturition 3.
Darifenacin, on the other hand, has been found to have a similar efficacy profile to solifenacin, but with a different pharmacokinetic profile 4.
Both drugs have been found to be well-tolerated, with the majority of side effects being mild to moderate in nature 3, 4.

A cost-utility analysis found that solifenacin was associated with the highest quality-adjusted life-year (QALY) gains for all three outcomes of interest (urgency, frequency, and incontinence) 5.
Solifenacin was found to be dominant relative to fesoterodine, tolterodine extended-release, and tolterodine immediate-release, and cost-effective relative to propiverine extended-release for urgency, frequency, and incontinence 5.

A meta-analysis comparing solifenacin and tolterodine found that the two drugs had similar efficacy and safety profiles, with no significant difference in daily micturition frequency, daily urgency episodes, daily incontinence episodes, and micturition volume per voiding 6.
However, tolterodine was found to decrease the constipation rate at 12 weeks compared to solifenacin 6.
There is limited data available for direct comparisons between solifenacin and darifenacin, highlighting the need for further studies to establish the differences between these drugs over longer periods of treatment 2.