Can Benadryl (diphenhydramine) cause urinary retention?

Diphenhydramine and Urinary Retention

Yes, Benadryl (diphenhydramine) can cause urinary retention due to its anticholinergic properties, and should be avoided in patients with existing urinary retention or prostatic hyperplasia.

Mechanism and Risk

Diphenhydramine, a first-generation antihistamine, exerts significant anticholinergic effects that can impair bladder contractility, leading to urinary retention. This occurs through:

• Blockade of muscarinic receptors in the detrusor muscle
• Inhibition of parasympathetic nervous system activity that normally facilitates bladder emptying
• Relaxation of the detrusor muscle while increasing urethral resistance

The FDA drug label for diphenhydramine explicitly warns against its use in patients with urinary problems, stating that patients should "ask a doctor before use if you have trouble urinating due to enlarged prostate gland" 1.

Risk Factors

Several factors increase the risk of diphenhydramine-induced urinary retention:

• Age: Elderly patients are particularly vulnerable due to age-related changes in bladder function
• Gender: Men with benign prostatic hyperplasia (BPH) have significantly higher risk 2
• Concomitant medications: Risk increases when combined with other medications with anticholinergic properties
• Dosage: Higher doses may increase risk, though the relationship is not strictly dose-dependent
• Route of administration: Intravenous administration may pose higher risk than oral 3

Clinical Evidence

The American Urological Association (AUA) guidelines explicitly state that "clinicians should not use anti-muscarinics in patients with narrow angle glaucoma unless approved by the treating ophthalmologist and should use anti-muscarinics with extreme caution in patients with impaired gastric emptying or a history of urinary retention" 4.

The guidelines further emphasize that "prior to initiation of anti-muscarinics, a patient at risk for urinary retention should receive clearance from a urologist" 4.

Management Considerations

If a patient develops urinary retention while taking diphenhydramine:

1. Discontinue the medication immediately
2. Bladder catheterization for prompt decompression if acute retention occurs
3. Consider alternative antihistamines with lower anticholinergic activity
4. Monitor post-void residual volume in at-risk patients who must use diphenhydramine

Prevention Strategies

To prevent diphenhydramine-induced urinary retention:

• Avoid use in patients with history of urinary retention or BPH
• Use lowest effective dose when the medication is necessary
• Consider second-generation antihistamines which have minimal anticholinergic effects
• Avoid combining with other medications that can cause urinary retention
• Monitor carefully if the patient has risk factors but requires diphenhydramine

Special Populations

Elderly Patients

Older adults are at higher risk due to:

• Age-related changes in bladder function
• Higher prevalence of BPH in older men
• Increased likelihood of polypharmacy with other anticholinergic medications 4

Men with BPH

Men with BPH have a substantially increased risk (adjusted OR 4.67) of developing acute urinary retention when using anticholinergic medications 5.

Alternative Antihistamines

For patients with urinary retention concerns, consider:

• Second-generation antihistamines (cetirizine, loratadine, fexofenadine)
• Non-sedating antihistamines with minimal anticholinergic effects
• Topical antihistamines when appropriate

Remember that even a single dose of diphenhydramine can potentially trigger urinary retention in susceptible individuals, as documented in case reports 3.