Tadalafil's Effect on the Internal Urethral Sphincter
Tadalafil relaxes the internal urethral sphincter by inhibiting phosphodiesterase type 5 (PDE5), which enhances nitric oxide-mediated smooth muscle relaxation in the prostate, bladder neck, and urethra, resulting in decreased urethral opening pressure and improved urinary flow.
Mechanism of Action on Urethral Sphincter Smooth Muscle
PDE5 inhibition by tadalafil increases cyclic GMP (cGMP) levels in smooth muscle cells of the prostate, bladder neck, and urethra, leading to smooth muscle relaxation 1.
The FDA label explicitly states that PDE5 is found in the smooth muscle of the prostate, bladder, and urethra, and tadalafil's inhibition of this enzyme results in smooth muscle relaxation in these tissues 1.
Tadalafil enhances nitric oxide-mediated vasodilation and smooth muscle relaxation by preventing the breakdown of cGMP, which is the key second messenger for smooth muscle relaxation 1.
Direct Evidence of Urethral Pressure Reduction
A 2024 randomized, double-blind, placebo-controlled crossover trial in healthy women demonstrated that a single 40 mg dose of tadalafil significantly reduced opening urethral pressure by 6.8 cmH₂O during resting conditions (95% CI, -11.8 to -1.9; p = 0.009) and by 8.8 cmH₂O during squeezing conditions (95% CI, -14.6 to -3.1; p = 0.005) 2.
This study provides direct urodynamic evidence that tadalafil reduces urethral sphincter tone through smooth muscle relaxation, rather than through vascular engorgement of the periurethral plexus 2.
Synergistic Effects with Alpha-Blockers on Neurogenic Contractions
A 2012 study demonstrated that tadalafil (30 nM) significantly inhibited tetrodotoxin-sensitive neurogenic contractions in human peripheral prostate tissue, indicating an effect on neurogenic smooth muscle tone 3.
When tadalafil was combined with tamsulosin (an alpha-blocker), there was enhanced inhibition of neurogenic contractions in both human prostate and bladder neck tissues compared to either agent alone 3.
However, the American Urological Association explicitly recommends against combining tadalafil 5 mg with alpha-blockers for LUTS/BPH because the combination provides no additional symptom benefit over monotherapy and significantly increases adverse events, particularly hypotension (Evidence Grade C, moderate recommendation) 4.
Clinical Implications for Lower Urinary Tract Symptoms
Tadalafil 5 mg once daily is FDA-approved for treating signs and symptoms of benign prostatic hyperplasia (BPH), which includes both storage symptoms (frequency, urgency, nocturia) and obstructive symptoms (incomplete emptying, weak stream, straining) 1.
The mechanism for reducing BPH symptoms involves relaxation of smooth muscle in the prostate, bladder neck, and their vascular supply, though the FDA label notes that the precise mechanism has not been fully established 1.
Tadalafil works by relaxing smooth muscle in the prostate and bladder, improving urinary flow and reducing storage symptoms, according to the American Urological Association 5.
Dosing and Clinical Efficacy
The only recommended dose for BPH/LUTS is tadalafil 5 mg once daily—higher doses do not provide additional benefit for urinary symptoms 5.
In clinical trials, tadalafil 5 mg once daily resulted in statistically significant improvements in total International Prostate Symptom Score (IPSS) compared to placebo (Study J: -4.8 vs -2.2, p<.001; Study K: -5.6 vs -3.6, p=0.004) 1.
Both irritative symptoms (frequency, urgency, nocturia) and obstructive symptoms (incomplete emptying, weak stream, straining) improved significantly with tadalafil therapy 4.
Critical Safety Considerations
Tadalafil is absolutely contraindicated with concurrent nitrate use (including sublingual nitroglycerin, long-acting nitrates, or recreational "poppers") due to the risk of potentially fatal hypotension 4, 6.
Tadalafil is also contraindicated with guanylate cyclase stimulators like riociguat because the combination can cause severe hypotension 6.
Assess cardiovascular fitness before prescribing tadalafil, and refer to cardiology if the patient cannot perform moderate physical activity or if cardiovascular risk is indeterminate 4, 6.
Comparison with Alpha-Blockers
Tamsulosin (an alpha-blocker) works primarily by blocking alpha-1 adrenergic receptors in the bladder neck and prostate, reducing smooth muscle tone through a different mechanism than tadalafil 4.
The American Urological Association states that daily 5 mg tadalafil and daily 0.4 mg tamsulosin provide comparable improvement in LUTS/BPH, with no statistically significant difference in IPSS reduction 4.
For patients with both LUTS/BPH and erectile dysfunction, tadalafil 5 mg once daily is the preferred first-line therapy because it simultaneously improves urinary symptoms and erectile function with a single agent 4.