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Urex Side Effects

Generic Name: methenamine

Note: This page contains side effects data for the generic drug methenamine. It is possible that some of the dosage forms included below may not apply to the brand name Urex.

It is possible that some side effects of Urex may not have been reported. These can be reported to the FDA here. Always consult a healthcare professional for medical advice.

For the Consumer

Applies to methenamine: oral suspension, oral tablet, oral tablet enteric coated

As well as its needed effects, methenamine (the active ingredient contained in Urex) may cause unwanted side effects that require medical attention.

If any of the following side effects occur while taking methenamine, check with your doctor immediately:

Less common
  • Skin rash
  • Blood in urine
  • lower back pain
  • pain or burning while urinating

Some methenamine side effects may not need any medical attention. As your body gets used to the medicine these side effects may disappear. Your health care professional may be able to help you prevent or reduce these side effects, but do check with them if any of the following side effects continue, or if you are concerned about them:

Less common
  • Nausea and vomiting

For Healthcare Professionals

Applies to methenamine: compounding powder, oral suspension, oral tablet


Methenamine (the active ingredient contained in Urex) is generally well tolerated. Minor side effects have been reported in less than 3.5% of patients.[Ref]


Dermatologic side effects have included rash and pruritus.[Ref]


Gastrointestinal side effects have included nausea, vomiting, diarrhea, anorexia, and abdominal cramping.[Ref]


Genitourinary side effects have included dysuria, hematuria, and urinary frequency. High doses have been associated with albuminuria, gross hematuria, bladder wall irritation, and painful/frequent micturition.[Ref]


Methenamine (the active ingredient contained in Urex) is contraindicated in patients with severe liver disease due to the risk of systemic acidosis.[Ref]

Hepatic side effects have rarely included transient elevations of AST (SGOT) and ALT (SGPT).[Ref]


Hypersensitivity reactions have included rashes in 1% to 4% of patients.[Ref]


Respiratory system side effects may occur in patients with a poor gag reflex or a swallow disorder. Methenamine oral suspension has a vegetable base, and should be used with caution in elderly patients and in those with swallow disorders to prevent lipid pneumonia. Long-term therapy with methenamine (the active ingredient contained in Urex) suspension has been associated with lipoid pneumonitis in elderly patients.[Ref]


Ocular side effects have included rare cases of anterior uveitis.[Ref]


Renal side effects have been more likely and more severe in patients with mild to moderate renal insufficiency. Methenamine (the active ingredient contained in Urex) is contraindicated in patients with severe renal insufficiency.[Ref]


1. "Product Information. Hiprex (methenamine)." Hoechst Marion-Roussel Inc, Kansas City, MO.

2. Gleckman R, Alvarez S, Joubert D, Matthews S "Drug therapy reviews: methenamine mandelate and methenamine hippurate." Am J Hosp Pharm 36 (1979): 1509-12

3. "Product Information. Mandelamine (methenamine)." Parke-Davis, Morris Plains, NJ.

4. Australian Drug Evaluation Committee "Adverse effects of drugs commonly used in the treatment of urinary tract infection." Med J Aust 1 (1972): 435-8

5. "Product Information. Urised (methenamine)." Polymedica Pharmaceuticals USA Inc, Woburn, MA.

6. Brumfitt W, Hamilton-Miller J, Gargan R, Cooper J, Smith GW "Long-term prophylaxis of urinary infections in women: comparative trial of trimethoprim, methenamine hippurate and topical povidone-iodine." J Urol 130 (1983): 1110-4

7. Brumfitt W, Cooper J, Hamilton-Miller J "Prevention of recurrent urinary infections in women: a comparative trial between nitrofurantoin and methenamine hippurate." J Urol 126 (1981): 71-4

8. Timmerman R, Schroer J "Lipoid pneumonia caused by methenamine mandelate suspension." JAMA 225 (1973): 1524-5

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