Methenamine is an anti-infective medication that is used to help prevent infections of the urinary tract. It is usually prescribed on a long-term basis for individuals with a history of repeated or chronic urinary tract infections.
Before you start taking this medication, check your urine with phenaphthazine paper or another test to see if it is acidic. Your urine must be acidic (pH 5.5 or below) for this medicine to work properly. Consult your health care professional about possible changes in your diet if necessary to increase the acidity of your urine (e.g., avoiding citrus fruits and juices, milk and other dairy products, antacids; eating more protein and foods such as cranberries and cranberry juice with added vitamin C, prunes, or plums).
The effects of methenamine in pregnancy have not been studied in either humans or animals. Individual case reports have not shown that this medication causes birth defects or other problems in humans.
Methenamine passes into breast milk but has not been reported to cause problems in nursing infants.
Possible Side Effects
Side effects that typically go away as your body adjusts to the medication and do not require medical attention unless they continue or are bothersome: nausea; vomiting.
Unusual side effects that should be reported immediately to your physician: skin rash.