Do metronidazole and amoxicillin cause urinary frequency?

Metronidazole and Amoxicillin as Causes of Urinary Frequency

Both metronidazole and amoxicillin can cause urinary frequency, though this is an uncommon side effect that is rarely reported in clinical literature. While neither drug lists urinary frequency as a common adverse effect in their FDA labels, there is evidence suggesting these medications can affect urinary patterns.

Metronidazole and Urinary Effects

Metronidazole is primarily eliminated through the kidneys, with 60-80% of the dose excreted in urine 1. This renal clearance pathway may contribute to urinary symptoms:

• Urine discoloration: Multiple case reports document urine discoloration (reddish or cola-colored) associated with metronidazole use 2, 3
• Metabolite excretion: Metronidazole produces metabolites including acetamide and N-(2-hydroxyethyl)oxamic acid that are excreted in urine 4
• Irritative effects: The excretion of the drug and its metabolites may cause irritation to the urinary tract, potentially leading to frequency

The pharmacokinetics of metronidazole show that less than 20% is excreted unchanged in urine, with the remainder as metabolites 5. These metabolites could potentially irritate the bladder mucosa, leading to urinary frequency in some patients.

Amoxicillin and Urinary Effects

Amoxicillin is also primarily eliminated by the kidneys and can affect urinary patterns:

• Crystalluria: The FDA label for amoxicillin notes that crystalluria can occur after overdosage, which could potentially cause urinary frequency 6
• Renal clearance: Amoxicillin is "substantially excreted by the kidney" according to its FDA label 6
• Irritation: The excretion of high concentrations of the drug may cause irritation to the urinary tract

Clinical Considerations

When evaluating urinary frequency in patients taking these medications:

• Rule out infection: Urinary frequency could be due to a urinary tract infection rather than a medication side effect
• Consider timing: Onset of symptoms in relation to medication initiation can help determine causality
• Evaluate alternatives: If urinary frequency is troublesome, consider alternative antimicrobials if clinically appropriate

Special Populations

• Renal impairment: Patients with decreased renal function may experience more urinary side effects due to altered drug clearance 1, 6
• Elderly patients: More likely to have decreased renal function and therefore may be at higher risk for urinary side effects 6

Management Approaches

If urinary frequency occurs:

1. Ensure adequate hydration to dilute urinary concentration of the drug and metabolites
2. Complete the prescribed course if the symptom is tolerable and the infection requires treatment
3. Consider alternative antibiotics if symptoms are severe or intolerable
4. Reassure patients about the transient nature of these effects, particularly with metronidazole-induced urine discoloration 3

While urinary frequency is not among the most commonly reported side effects for either medication, clinicians should be aware of this potential adverse effect when prescribing these antibiotics, particularly in patients with pre-existing urinary symptoms or conditions.