Can Hiprex Cause Loose Stools?
Hiprex (methenamine hippurate) is not documented to cause loose stools or diarrhea as a side effect. According to the FDA drug label, the adverse reactions to methenamine are mild, reversible, and occur infrequently, primarily consisting of gastrointestinal intolerance and skin reactions 1.
Evidence from Clinical Studies
The safety profile of methenamine hippurate has been well-characterized across multiple clinical trials and real-world studies:
Gastrointestinal side effects are minimal: Research spanning decades consistently shows that adverse reactions are uncommon. When gastrointestinal symptoms do occur, they typically manifest as nausea or taste complaints, not diarrhea 2.
Large-scale clinical experience: A 2019 study in renal transplant recipients found that only 1 patient experienced nausea and 1 was intolerant to the medication—no cases of loose stools or diarrhea were reported 3. Similarly, a pediatric study from 1978 noted that "a few girls complained of the taste but no side-effects were observed" 4.
Recent trials confirm safety: The 2025 ImpresU trial, a triple-blind randomized controlled trial in 289 older women, found no important differences in complications between methenamine hippurate and placebo groups, with no mention of diarrhea as a side effect 5.
Mechanism of Action Context
Methenamine hippurate works by releasing formaldehyde in acidic urine, providing antibacterial activity locally in the urinary tract 1. This mechanism does not affect intestinal motility or secretion, which distinguishes it from medications that commonly cause diarrhea. The hippuric acid component helps maintain urine acidity and has minimal systemic effects.
Clinical Implications
If a patient on Hiprex develops loose stools, look for alternative causes:
- Concurrent medications (antibiotics, laxatives, proton pump inhibitors)
- Underlying gastrointestinal conditions (IBS, inflammatory bowel disease)
- Dietary factors (lactose intolerance, high-osmolar supplements)
- Infectious causes (bacterial, viral, or parasitic gastroenteritis)
The extensive clinical experience with methenamine hippurate across diverse patient populations—including elderly women, children, renal transplant recipients, and those with urinary tract abnormalities—consistently demonstrates that loose stools are not a recognized adverse effect of this medication 2, 3, 4, 6, 5, 7.