Yes, Mirtazapine Can Cause Urinary Retention, But it is Rare
While urinary retention is not a common side effect of mirtazapine, it is a documented, albeit rare, adverse event. Several case reports detail instances of individuals developing urinary retention shortly after starting or increasing their mirtazapine dosage. This adverse effect is particularly noted in vulnerable populations, such as elderly male patients who may have pre-existing conditions like benign prostatic hyperplasia (BPH). Clinicians must be aware of this potential side effect, especially when treating at-risk individuals, and consider it as a differential diagnosis for new or worsening urinary symptoms.
The Pharmacological Mechanism Behind Urinary Retention
Unlike some older antidepressants with strong anticholinergic properties that block bladder muscle contraction, mirtazapine's mechanism for causing urinary retention is different. The primary cause is related to its effect on the noradrenergic system. Mirtazapine acts as a presynaptic alpha-2 antagonist, which increases the release of the neurotransmitter noradrenaline (NE). This increase in NE can lead to the contraction of the smooth muscle of the urethral sphincter, making it difficult or impossible to void the bladder completely, or even at all.
Additionally, higher doses of mirtazapine may amplify this noradrenergic effect, as one case study illustrated a dose-dependent urinary retention in a female patient when her dosage was increased from 7.5 mg to 15 mg. The symptoms subsided immediately upon returning to the lower dose, further supporting the dose-related nature of this side effect.
Risk Factors for Mirtazapine-Induced Urinary Retention
Several factors can increase an individual's susceptibility to mirtazapine-induced urinary retention:
- Elderly Male Patients with BPH: Older males, especially those with an enlarged prostate, are significantly more vulnerable. A 2025 case report described a 70-year-old male with BPH who developed acute urinary retention after starting mirtazapine.
- Underlying Urinary Issues: Any pre-existing bladder dysfunction or urinary tract problems can be exacerbated by mirtazapine's noradrenergic effects.
- Concomitant Medications: Taking other medications that can affect urinary function, such as those with anticholinergic effects or other agents that affect adrenergic receptors, may increase the risk.
- Dosage: As demonstrated in clinical cases, increasing the dose of mirtazapine may increase the risk of urinary retention in some individuals.
Managing Mirtazapine-Induced Urinary Retention
If a patient experiences urinary retention while on mirtazapine, prompt medical attention is necessary. In acute cases, a healthcare provider will likely need to insert a urinary catheter to relieve bladder pressure and drain the urine. Following this, the following steps are generally considered:
- Dosage Adjustment or Cessation: The most straightforward management approach is to reduce or discontinue the mirtazapine dosage. In reported cases, symptoms of urinary retention often resolve quickly after the medication is stopped.
- Review All Medications: The healthcare provider will evaluate all other medications being taken to identify any potential drug-drug interactions that might contribute to the issue.
- Address Underlying Conditions: If conditions like BPH are contributing factors, a urologist may need to get involved to manage the underlying issue.
- Switch Antidepressants: A different class of antidepressant, such as a selective serotonin reuptake inhibitor (SSRI), may be considered if mirtazapine is the confirmed cause of the urinary retention.
Comparison of Antidepressant-Induced Urinary Retention
While mirtazapine can cause urinary retention, it is important to understand how its risk profile compares to other classes of antidepressants. The mechanisms and severity can vary significantly.
| Antidepressant Class | Typical Risk Profile for Urinary Retention | Primary Pharmacological Mechanism | Key Characteristics |
|---|---|---|---|
| Mirtazapine (NaSSA) | Rare, but documented risk. | Alpha-2 antagonism increasing noradrenaline, causing sphincter contraction. | Considered safer for urinary side effects than older TCAs. Has been reported to reverse SSRI-induced retention. |
| SSRIs (e.g., citalopram) | Generally low risk, but documented cases exist. | Central facilitation of storage pathways via serotonergic effects. | May cause urinary hesitancy. Mirtazapine augmentation can reverse this effect in some cases. |
| SNRIs (e.g., duloxetine) | Can induce urinary retention due to noradrenergic effects. | Increased noradrenergic tone stimulates α1-adrenoceptors in the internal sphincter. | Duloxetine is sometimes used to treat stress incontinence due to its effect on the urethral sphincter. |
| TCAs (e.g., amitriptyline) | Higher risk due to potent anticholinergic effects. | Strong muscarinic receptor antagonism impairs bladder muscle contraction. | Anticholinergic effects can also cause dry mouth and constipation. |
What to Do if You Suspect Urinary Retention
If you are taking mirtazapine and experience signs of urinary retention, it is crucial to contact your healthcare provider immediately. This is not a symptom to ignore, as delayed treatment can lead to complications such as bladder damage or kidney issues. Signs and symptoms may include:
- Inability to urinate, or difficulty starting urination.
- A weak stream or a feeling of incomplete bladder emptying.
- Pain or discomfort in the lower abdomen or pelvic region.
- Increased frequency of urination, which can occur paradoxically.
Conclusion
While mirtazapine is generally considered safe regarding urinary side effects and is often used as a preferred alternative to antidepressants with more prominent anticholinergic effects, the possibility of urinary retention remains. The mechanism is distinct, primarily involving its noradrenergic properties and sphincter constriction, not its weak anticholinergic action. Awareness of this rare risk is vital for both patients and clinicians, especially in high-risk groups such as elderly males with BPH. Patients experiencing difficulty urinating should seek immediate medical attention, as prompt management, including potential dose reduction or discontinuation, can resolve the issue effectively. For further information on antidepressant side effects, consult reliable resources like the Mayo Clinic.
