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Skeletal Muscle Relaxants

Skeletal muscle relaxants (SMRs) are used in urology to relax three different anatomic targets — the external urethral sphincter (in detrusor sphincter dyssynergia), the pelvic floor musculature (in high-tone pelvic floor dysfunction and myofascial pelvic pain), and indirectly the detrusor (via spinal-cord-acting agents). The dominant urologic muscle-relaxing therapy — botulinum toxin A at the detrusor or sphincter — is covered in its own article; this page focuses on the systemically administered SMRs (baclofen, dantrolene, diazepam, cyclobenzaprine), where the evidence base is older, less consistent, and increasingly displaced by injection-based therapy.[1][2]

For agents covered separately:

Anatomic targets​

TargetMuscle typeAgent classes
External urethral sphincterStriatedBaclofen (oral, intrathecal); dantrolene; intrasphincteric BoNT-A
Pelvic floor (levator ani, obturator internus)StriatedDiazepam (oral, vaginal); cyclobenzaprine (off-label); BoNT-A injection
Detrusor (indirectly)SmoothIntrathecal baclofen (spinal-cord effect on EUS motoneurons and detrusor reflexes); intradetrusor BoNT-A
Bladder neck / prostate / ureterSmoothAlpha-blockers (covered separately)

Baclofen — for DSD and neurogenic spasticity​

A GABA-B receptor agonist used principally for limb spasticity in spinal-cord pathology, with secondary benefits on detrusor sphincter dyssynergia (DSD) and detrusor hyperreflexia.[3]

Mechanism in urology​

  • Oral: central GABA-B agonism depresses pudendal-to-pudendal nerve reflex via spinal cord presynaptic hyperpolarization → reduced external urethral sphincter resistance[4]
  • Intrathecal (ITB): direct lumbosacral GABA-B activation in the nucleus of Onuf inhibits EUS motoneurons → urethral relaxation during detrusor contraction. The dose required for urethral inhibition is lower than the dose that inhibits detrusor contractions, providing a therapeutic window[5][6]

Oral baclofen evidence​

In 25 SCI patients with DSD, oral baclofen reduced post-void residual significantly in 73% of cases within ~5 weeks and was proposed as an alternative to external sphincterotomy for postural DSD.[4] The principal limitation is poor blood–brain barrier penetration: tolerable oral doses often produce sedation and weakness without reaching effective spinal cord concentrations for DSD.[3]

Intrathecal baclofen for genitourinary effects​

In a prospective blinded study of continuous ITB in 10 patients with severe SCI spasticity:[5]

  • Eliminated uninhibited bladder contractions in all patients with irritative voiding and urge incontinence
  • Abolished DSD in 40%
  • Achieved 72% increase in bladder capacity and 16% improvement in compliance (in non-cervical SCI patients)
  • 1 of 3 patients with an indwelling catheter converted to clean intermittent self-catheterization

Clinical implication for urologists: patients with ITB pumps implanted for limb spasticity should be re-evaluated for bladder function — ITB frequently produces secondary urologic improvements (reduced DSD, increased capacity, decreased incontinence).[3][5]

Adverse effects relevant to LUT[7]​

Sedation, weakness, dizziness, headache; LUT-specific: urinary frequency (2–6%) and rarely enuresis, urinary retention, dysuria, impotence, ejaculatory failure, nocturia. Abrupt withdrawal — particularly of intrathecal baclofen — can produce a life-threatening hyperthermia / spasticity / rhabdomyolysis syndrome.

Dantrolene — for external sphincter hypertonicity (largely historical)​

The only peripherally acting oral SMR — interferes with calcium release from the sarcoplasmic reticulum to reduce excitation–contraction coupling in skeletal muscle.[8][9]

Selectivity matters​

Dantrolene acts on striated muscle only: it has no significant effect on detrusor smooth-muscle contractility, which makes it conceptually attractive for isolated EUS hypertonicity that spares the detrusor.[10]

Evidence​

  • In 15 SCI patients with PVR > 150 mL secondary to EUS hypertonicity (urethral closure pressures > 100 cm H₂O), 8 of 15 (53%) benefited from dantrolene — successful patients had a mean urethral pressure decrease of 77 cm H₂O (49%). However, 5 of the 8 responders required doses up to 600 mg/day, in the hepatotoxic range[8]
  • Early reports also showed efficacy for non-neurogenic external sphincter spasm causing functional outlet obstruction[9]
  • Reported as a salvage option for refractory neurogenic detrusor overactivity in cerebral palsy when combined with antimuscarinics and β3-agonists[11]

Limitations​

  • High doses often required (up to 600 mg/day) → significant hepatotoxicity (boxed warning)[12]
  • Generalized muscle weakness limits functional utility, particularly in incomplete SCI where ambulation is preserved
  • Largely supplanted by intrasphincteric BoNT-A, which provides targeted relaxation without systemic weakness
  • Detrusor hyporeflexia frequently coexists and contributes to treatment failure

Current role​

Rarely used in modern urologic practice for sphincter dysfunction. Niche role as adjunctive therapy for refractory neurogenic detrusor overactivity in select non-SCI populations (e.g., cerebral palsy).[11]

Diazepam — for high-tone pelvic floor dysfunction​

The most-studied SMR for myofascial pelvic pain and high-tone pelvic floor disorders, available as oral tablets and as compounded vaginal suppositories (typically 10 mg).[1][13]

Pharmacokinetics — vaginal vs oral​

Compounded 10 mg vaginal diazepam has a distinctive PK profile: peak concentration 31.0 ng/mL at ~3.1 h, bioavailability 70.5%, and a terminal half-life of 82 hours — markedly lower peak levels but a longer half-life than oral. This profile favors intermittent use (allowing PT and intimacy on alternating days) rather than fixed daily dosing.[14]

Evidence — mixed at best​

StudyDesignResult
Stone systematic review[13]5 studies (2 observational, 3 RCTs)Observational studies report 71–96% subjective improvement; RCTs found no significant difference in FSFI or VAS pain scores
Crisp triple-blinded RCT (n = 21)[15]10 mg vaginal diazepam nightly × 4 wk vs placeboNo improvement in resting pelvic-floor EMG or subjective outcomes
Murina RCT — vestibulodynia[16]Vaginal diazepam + TENS vs placebo + TENSSignificantly improved dyspareunia and ability to relax PF muscles after contraction
Rogalski retrospective (n = 26)[17]Vaginal diazepam + PT + trigger-point injectionsImproved pelvic-floor muscle tone at rest, squeeze, and relaxation phases

Practical interpretation: intravaginal diazepam alone appears insufficient for high-tone pelvic floor dysfunction; it may be useful as an adjunct to multimodal therapy (pelvic floor PT, trigger-point injections, TENS), but the high-quality RCT evidence does not support monotherapy.[1][13][15]

Oral diazepam​

Used for pelvic-floor spasm and chronic pelvic pain but limited by sedation, dependence potential, and cognitive effects. In vitro work shows diazepam has no direct effect on human bladder smooth-muscle contractility — its urologic effects are centrally mediated.[18]

Cyclobenzaprine and Other Centrally Acting Antispasmodics​

A tricyclic-related agent prescribed off-label for myofascial pelvic pain and CP/CPPS, with very limited supporting evidence.[1][19][20]

  • No high-quality studies demonstrate improvement in pelvic-pain scales with cyclobenzaprine specifically
  • Anticholinergic adverse effects (dry mouth, urinary retention, sedation) may be counterproductive in patients with concurrent voiding dysfunction[19]
  • Other centrally acting agents (methocarbamol, metaxalone, carisoprodol) have no established urologic role
  • The ACOG Chronic Pelvic Pain Practice Bulletin notes that SMRs have limited high-quality evidence for chronic myofascial pelvic pain and should be used as part of multimodal management rather than as primary therapy[2]

For the broader pelvic-pain pharmacotherapy framework, see Antispasmodics.

SMRs for Postoperative Urinary Retention​

Methocarbamol, meprobamate, chlormezanone, diazepam, and carisoprodol have all been studied for postoperative urinary retention (POUR) on the rationale that they reduce reflex pelvic-floor and EUS contracture (or anxiety-related voiding inhibition). The Cochrane review found insufficient evidence to support routine use of any drug — including SMRs — for the treatment of POUR. Alpha-blockers have the most evidence in this setting, though even they are not definitively proven.[21]

Evidence Summary​

AgentSettingEvidenceBottom line
Oral baclofenDSD in SCI73% PVR reduction in 25 patients[4]Modest benefit; sedation often limits dose
Intrathecal baclofenDSD + detrusor hyperreflexia in SCIEliminates uninhibited contractions; abolishes DSD in 40%; +72% capacity[5]Implanted for limb spasticity; secondary urologic gains
DantroleneEUS hypertonicity in SCI53% benefit but 5/8 responders need ≥ 600 mg/day[8]Largely replaced by intrasphincteric BoNT-A
Vaginal diazepamHigh-tone pelvic floor dysfunctionStone meta: no benefit in RCTs; helpful adjunct in some series[13][15][16][17]Adjunct only — not monotherapy
CyclobenzaprineMyofascial CPPNo high-quality urologic data[1]Off-label; consider only as multimodal adjunct
POUR (any SMR)Postoperative urinary retentionCochrane: insufficient evidence[21]Not recommended as primary therapy
Functional BOO in NLUTD (any approach)Surgical / medical managementCochrane: limited evidence for any single approach[22]Multimodal, individualized strategy

Clinical Positioning​

  • Intrasphincteric and intradetrusor BoNT-A dominate modern urologic muscle-relaxing therapy — see Botulinum toxin for the principal pharmacologic option for DSD, NDO, and OAB
  • Baclofen has a defensible role in the SCI patient with both limb spasticity and DSD — particularly intrathecal, which produces secondary urologic benefits beyond its limb-spasticity indication
  • Dantrolene is largely historical — consider only when sphincter-targeted BoNT-A is unavailable or contraindicated, and counsel about hepatotoxicity at the doses required
  • Vaginal diazepam is an adjunct, not monotherapy — pair with pelvic floor PT, trigger-point injections, and TENS in high-tone pelvic floor dysfunction
  • Cyclobenzaprine and other antispasmodics lack high-quality urologic evidence — and their anticholinergic burden can worsen voiding dysfunction
  • No SMR is routinely indicated for postoperative urinary retention[21]
  • The urologic adverse effects of systemically administered SMRs (urinary retention with baclofen and dantrolene; anticholinergic effects with cyclobenzaprine) must be considered when these agents are prescribed for non-urologic indications in patients with pre-existing LUTS[7][12][19]

See Also​

References​

1. Flatow V, Uy-Kroh J, Carey ET, Ascher-Walsh C, Khalil S. "Skeletal muscle relaxants for the treatment of myofascial pelvic pain and high tone pelvic floor disorders." Curr Opin Obstet Gynecol. 2023;35(4):311–315. doi:10.1097/GCO.0000000000000894

2. American College of Obstetricians and Gynecologists. "Chronic pelvic pain: ACOG Practice Bulletin, Number 218." Obstet Gynecol. 2020;135(3):e98–e109. doi:10.1097/AOG.0000000000003716

3. Verduzco-Gutierrez M, Raghavan P, Pruente J, et al. "AAPM&R consensus guidance on spasticity assessment and management." PM R. 2024;16(8):864–887. doi:10.1002/pmrj.13211

4. Leyson JF, Martin BF, Sporer A. "Baclofen in the treatment of detrusor-sphincter dyssynergia in spinal cord injury patients." J Urol. 1980;124(1):82–84. doi:10.1016/s0022-5347(17)55307-0

5. Steers WD, Meythaler JM, Haworth C, Herrell D, Park TS. "Effects of acute bolus and chronic continuous intrathecal baclofen on genitourinary dysfunction due to spinal cord pathology." J Urol. 1992;148(6):1849–1855. doi:10.1016/s0022-5347(17)37048-9

6. Miyazato M, Sasatomi K, Hiragata S, et al. "Suppression of detrusor-sphincter dyssynergia by GABA-receptor activation in the lumbosacral spinal cord in spinal cord-injured rats." Am J Physiol Regul Integr Comp Physiol. 2008;295(1):R336–R342. doi:10.1152/ajpregu.90315.2008

7. US Food and Drug Administration. Baclofen — prescribing information. Updated 2024-04-17.

8. Hackler RH, Broecker BH, Klein FA, Brady SM. "A clinical experience with dantrolene sodium for external urinary sphincter hypertonicity in spinal cord injured patients." J Urol. 1980;124(1):78–81. doi:10.1016/s0022-5347(17)55305-7

9. Murdock MM, Sax D, Krane RJ. "Use of dantrolene sodium in external sphincter spasm." Urology. 1976;8(2):133–137. doi:10.1016/0090-4295(76)90340-x

10. Harris JD, Benson GS. "Effect of dantrolene sodium on canine bladder contractility." Urology. 1980;16(2):229–231. doi:10.1016/0090-4295(80)90095-3

11. Bulchandani S, Toozs-Hobson P, Kennedy A, Sturman S. "Dantrolene an unusual option for detrusor overactivity: observations of a patient with cerebral palsy." Eur J Obstet Gynecol Reprod Biol. 2015;194:255–256. doi:10.1016/j.ejogrb.2015.08.012

12. US Food and Drug Administration. Dantrolene Sodium — prescribing information. Updated 2024-12-22.

13. Stone RH, Abousaud M, Abousaud A, Kobak W. "A systematic review of intravaginal diazepam for the treatment of pelvic floor hypertonic disorder." J Clin Pharmacol. 2020;60(Suppl 2):S110–S120. doi:10.1002/jcph.1775

14. Larish AM, Dickson RR, Kudgus RA, et al. "Vaginal diazepam for nonrelaxing pelvic floor dysfunction: the pharmacokinetic profile." J Sex Med. 2019;16(6):763–766. doi:10.1016/j.jsxm.2019.03.003

15. Crisp CC, Vaccaro CM, Estanol MV, et al. "Intra-vaginal diazepam for high-tone pelvic floor dysfunction: a randomized placebo-controlled trial." Int Urogynecol J. 2013;24(11):1915–1923. doi:10.1007/s00192-013-2108-9

16. Murina F, Felice R, Di Francesco S, Oneda S. "Vaginal diazepam plus transcutaneous electrical nerve stimulation to treat vestibulodynia: a randomized controlled trial." Eur J Obstet Gynecol Reprod Biol. 2018;228:148–153. doi:10.1016/j.ejogrb.2018.06.026

17. Rogalski MJ, Kellogg-Spadt S, Hoffmann AR, Fariello JY, Whitmore KE. "Retrospective chart review of vaginal diazepam suppository use in high-tone pelvic floor dysfunction." Int Urogynecol J. 2010;21(7):895–899. doi:10.1007/s00192-009-1075-7

18. Ayyat FM, Lloyd LK, Kuhlemeier KV. "Effect of skeletal muscle relaxants on bladder smooth muscle." J Urol. 1984;132(2):372–375. doi:10.1016/s0022-5347(17)49630-3

19. US Food and Drug Administration. Cyclobenzaprine Hydrochloride — prescribing information. Updated 2025-06-23.

20. Pena VN, Engel N, Gabrielson AT, Rabinowitz MJ, Herati AS. "Diagnostic and management strategies for patients with chronic prostatitis and chronic pelvic pain syndrome." Drugs Aging. 2021;38(10):845–886. doi:10.1007/s40266-021-00890-2

21. Buckley BS, Lapitan MC. "Drugs for treatment of urinary retention after surgery in adults." Cochrane Database Syst Rev. 2010;(10):CD008023. doi:10.1002/14651858.CD008023.pub2

22. Utomo E, Groen J, Blok BF. "Surgical management of functional bladder outlet obstruction in adults with neurogenic bladder dysfunction." Cochrane Database Syst Rev. 2014;(5):CD004927. doi:10.1002/14651858.CD004927.pub4