Methenamine, an antibiotic, eliminates bacteria that cause urinary tract infections. It usually is used on a long-term basis to treat chronic infections and to prevent recurrence of infections. Antibiotics will not work for colds, flu, or other viral infections.
Methenamine comes as a tablet and a liquid to take by mouth. It usually is taken either two times a day (every 12 hours) or four times a day (after meals and at bedtime). Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand. Take methenamine exactly as directed. Do not take more or less of it or take it more often than prescribed by your doctor.
Summary from MedlinePlus.gov, click for more information.