Can melatonin cause urinary retention, particularly in older adults with a history of urinary problems or benign prostatic hyperplasia (BPH)?

Melatonin Does Not Cause Urinary Retention

Melatonin does not cause urinary retention and may actually improve lower urinary tract symptoms, particularly nocturia, in older adults with BPH. The available evidence demonstrates that melatonin increases bladder capacity and reduces urinary frequency through central nervous system mechanisms, making it a potentially beneficial rather than harmful agent for patients with urinary problems 1, 2, 3.

Mechanism of Action on the Urinary System

Melatonin exerts favorable effects on bladder function through multiple pathways:

• Melatonin increases bladder capacity via GABAergic receptors in the brain, with studies showing dose-dependent increases of 27-64% in bladder capacity without affecting bladder contraction pressure 2.
• The hormone decreases urine volume independently of antidiuretic hormone, suggesting a direct central nervous system effect on micturition control 2.
• In animal models, melatonin counteracts age-related detrusor overactivity, restoring bladder function parameters to those of younger animals by reducing oxidative stress 4.

Clinical Evidence in BPH Patients

The research specifically addressing older men with BPH and urinary symptoms shows therapeutic benefit, not harm:

• A randomized controlled trial in men with urodynamically confirmed bladder outflow obstruction found melatonin 2 mg reduced nocturia episodes by 0.32 per night (compared to 0.05 with placebo, p=0.07) and significantly improved nocturia-related bother scores (p=0.008) 1.
• The nocturia responder rate (≥0.5 episode reduction) was significantly higher with melatonin versus placebo (p=0.04), with no adverse effects on daytime urinary frequency, maximum flow rate, or post-void residual volume 1.
• A 2022 randomized trial demonstrated that adding melatonin 3 mg to tamsulosin reduced nocturia likelihood by 2.39 times (95% CI: 1.07-5.32, p=0.033) and urinary frequency by 2.59 times (95% CI: 1.15-5.84, p=0.021) compared to tamsulosin alone 3.

Guideline Context and Safety Profile

Major sleep medicine and geriatric guidelines do not identify urinary retention as a concern with melatonin:

• The American Academy of Sleep Medicine's 2017 guideline on chronic insomnia makes no mention of urinary retention as an adverse effect of melatonin, though it notes limited efficacy data for sleep outcomes 5.
• The American Geriatrics Society's 2009 recommendations on sleep disorders in older persons do not list urinary retention among melatonin's side effects, despite extensive discussion of medications that can worsen urinary problems in this population 5.
• AUA guidelines on BPH management identify multiple medication classes that worsen urinary retention (anticholinergics, decongestants, antihistamines) but do not include melatonin 5.

Critical Distinction from Problematic Medications

It is essential to differentiate melatonin from medications that genuinely cause urinary retention in older adults with BPH:

• Anticholinergic medications (including first-generation antihistamines like diphenhydramine) directly impair detrusor contractility and should be avoided or held in patients with urinary retention 6, 7.
• Medications with alpha-adrenergic effects (like duloxetine) increase urethral sphincter tone and bladder outlet resistance, directly opposing therapeutic goals in BPH 8.
• Melatonin operates through entirely different mechanisms (GABAergic and circadian pathways) that do not involve anticholinergic or alpha-adrenergic effects 2.

Practical Clinical Application

For older adults with BPH or urinary problems considering melatonin:

• Melatonin 2-3 mg at bedtime can be safely used and may provide symptomatic benefit for nocturia, the most bothersome urinary symptom affecting quality of life in this population 1, 3.
• No baseline urodynamic testing or post-void residual monitoring is required specifically for melatonin initiation, unlike medications that genuinely increase retention risk 8, 1.
• Melatonin can be combined with standard BPH therapies (alpha-blockers, 5-alpha reductase inhibitors) without concern for additive urinary retention risk 3.

Common Pitfall to Avoid

Do not confuse melatonin with sedating antihistamines that have anticholinergic properties. While both are used for sleep, diphenhydramine and similar agents carry genuine urinary retention risk through anticholinergic mechanisms, whereas melatonin does not 5, 7.