Hiprex (Methenamine Hippurate)

Hiprex is indicated for the prevention of recurrent urinary tract infections (UTIs) in patients with a history of multiple episodes. It is used when long‑term antimicrobial prophylaxis is desired and when urine pH is sufficiently acidic to allow drug activation.

• Adults with a documented pattern of two or more UTIs within six months or three or more within a year.
• Patients who have completed a course of curative antibiotics and require ongoing protection.
• Individuals with uncomplicated cystitis who prefer a non‑antibiotic prophylactic option.
• Situations where bacterial resistance to standard antibiotics is a concern.
• Use in conjunction with measures that maintain urine acidity, such as dietary modification or vitamin C supplementation.

• Methenamine hippurate is a pro‑drug that is excreted unchanged in the urine. In an acidic urinary environment (pH ≤ 6), it hydrolyzes to release formaldehyde and ammonia. The formaldehyde acts as a broad‑spectrum antibacterial agent, disrupting microbial proteins and nucleic acids, thereby inhibiting bacterial growth within the bladder and urethra. Because the conversion occurs locally, systemic exposure is minimal. The resulting ammonia also helps to maintain urinary acidity, supporting continued drug activation.

Adverse reactions to Hiprex are generally uncommon, but they can be categorized by frequency and severity.

Common

• Nausea or mild stomach upset.
• Dyspepsia or heartburn.
• Metallic taste in the mouth.
• Mild headache or dizziness.
• Transient increase in urinary frequency.
• Dry mouth or altered taste sensation.
• Transient skin flushing.

Serious

• Severe allergic reactions such as rash, itching, or swelling.
• Acute interstitial nephritis presenting with flank pain and decreased urine output.
• Hematuria or blood in the urine unrelated to infection.
• Liver enzyme elevations indicating possible hepatic involvement.
• Seizures reported rarely in patients with severe renal impairment.
• Anaphylactic shock requiring immediate medical attention.
• Kidney failure in patients with pre‑existing renal disease.

Precautions for Hiprex focus on patient selection, renal function, urine acidity, and monitoring for allergic reactions.

• Assess renal function; the drug is ineffective and may accumulate in patients with severe renal impairment (creatinine clearance < 15 mL/min).
• Verify that urine pH is ≤ 6.0 before initiating therapy, as higher pH reduces drug activation.
• Use caution in patients with a history of hypersensitivity to methenamine or related compounds.
• Monitor for signs of infection recurrence, which may indicate inadequate prophylaxis.
• Avoid concurrent use of alkalinizing agents (e.g., sodium bicarbonate) that raise urinary pH.
• Counsel patients to maintain adequate hydration to support urinary excretion.

Hiprex may interact with other medicines that alter urine pH or affect renal excretion, and certain drugs can increase the risk of adverse reactions.

Avoid

• Alkalinizing agents such as sodium bicarbonate, potassium citrate, or antacids that raise urinary pH, which can prevent methenamine activation.
• High‑dose vitamin C supplements that significantly increase urinary alkalinity.

Use with caution

• Loop diuretics (e.g., furosemide) that may affect renal function and urine concentration.
• Non‑steroidal anti‑inflammatory drugs (NSAIDs) which can impair renal perfusion.
• Other urinary antiseptics or antibiotics, as combined use may increase the risk of toxicity or mask infection signs.