Methenamine Side Effects

It is possible that some side effects of methenamine 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

Along with its needed effects, methenamine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor immediately if any of the following side effects occur while taking methenamine:

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

Some side effects of methenamine may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects. Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:

Less common
  • Nausea and vomiting

For Healthcare Professionals

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


Methenamine is generally well tolerated. Minor side effects have been reported in less than 3.5% of patients.


Dermatologic side effects have included rash and pruritus.


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


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.


Methenamine is contraindicated in patients with severe liver disease due to the risk of systemic acidosis.

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


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


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 suspension has been associated with lipoid pneumonitis in elderly patients.


Ocular side effects have included rare cases of anterior uveitis.


Renal side effects have been more likely and more severe in patients with mild to moderate renal insufficiency. Methenamine is contraindicated in patients with severe renal insufficiency.

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