Nausea, vomiting, diarrhea, abdominal cramps, and loss of appetite may occur. If any of these effects last or get worse, tell your doctor or pharmacist promptly.

Painful or difficult urination may occur with methenamine, although less often. Report these side effects to your doctor, who may need to lower your dose of methenamine or change your treatment to make your urine less acidic.

Remember that this medication has been prescribed because your doctor has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects.

Taking higher than recommended doses of methenamine may cause bladder irritation, painful/frequent urination, and bloody/pink urine. Tell your doctor right away if you have any of these symptoms.

Tell your doctor right away if you have any serious side effects, including: mouth sores, unusual headache, ringing in the ears, muscle cramps, swelling of the arms/legs.

A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing.

This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist.

In the US -

Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch.

In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.