Methenamine is an antibacterial medicine used to treat or prevent bladder infections in adults and children at least 6 years old.

Methenamine may also be used for purposes not listed in this medication guide.

Use only as directed. Tell your doctor if you use other medicines or have other medical conditions or allergies.

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.

Swallow the coated tablets whole. Do not crush or break them. Take the tablets with a full glass of water or with food. Shake the liquid well before each use to mix the drug evenly.

Use exactly as directed on the label, or as prescribed by your doctor.

Methenamine works best when your urine has a low pH and is acidic. You can measure your pH levels at home using urine test strips purchased from a pharmacy. Your doctor may occasionally test your urine to make sure an infection has cleared.

Learn about the foods to eat or avoid to help keep your urine acidic. Your doctor may also prescribe other medicines. Drink plenty of liquids.

Follow your doctor's diet and medication instructions very closely.

Keep using this medicine even if your symptoms quickly improve. Skipping doses could make your infection resistant to medication. Methenamine will not treat a viral infection (flu or a common cold).

This medicine can affect the results of certain medical tests. Tell any doctor who treats you that you are using methenamine.

Do not use methenamine to treat any condition that has not been checked by your doctor.

Do not share this medicine with another person, even if they have the same symptoms you have.

Store at room temperature away from moisture, heat, and light. Keep the bottle tightly closed when not in use.

You should not use methenamine if you are allergic to it, or if you have:

• kidney disease;

• severe liver disease; or

• if you are dehydrated.

Ask a doctor or pharmacist if methenamine is safe to use if you've ever had liver problems.

Ask a doctor before using this medicine if you are pregnant.

Do not breastfeed.

Take the medicine as soon as you can, but skip the missed dose if it is almost time for your next dose. Do not take two doses at one time.

Seek emergency medical attention or call the Poison Help line at 1-800-222-1222.

Keep this medication in the container it came in, tightly closed, and out of reach of children. Store it at room temperature and away from excess heat and moisture (not in the bathroom).

Unneeded medications should be disposed of in special ways to ensure that pets, children, and other people cannot consume them. However, you should not flush this medication down the toilet. Instead, the best way to dispose of your medication is through a medicine take-back program. Talk to your pharmacist or contact your local garbage/recycling department to learn about take-back programs in your community. See the FDA's Safe Disposal of Medicines website (http://goo.gl/c4Rm4p) for more information if you do not have access to a take-back program.

It is important to keep all medication out of sight and reach of children as many containers (such as weekly pill minders and those for eye drops, creams, patches, and inhalers) are not child-resistant and young children can open them easily. To protect young children from poisoning, always lock safety caps and immediately place the medication in a safe location – one that is up and away and out of their sight and reach. http://www.upandaway.org

Methenamine may cause an upset stomach. Take methenamine with food or milk.

Avoid taking extra methenamine, or you could have side effects such as bladder irritation, pain, increased urination, and blood in the urine.

Get emergency medical help if you have signs of an allergic reaction: hives; difficult breathing; swelling of your face, lips, tongue, or throat.

Methenamine may cause serious side effects. Call your doctor at once if you have:

• increased urination;

• painful urination;

• blood in your urine; or

• swelling, shortness of breath.

Common side effects of methenamine may include:

• painful urination;

• nausea, vomiting; or

• rash, itching.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

Other drugs may affect methenamine, especially a sulfa drug. Tell your doctor about all other medicines you use. This includes prescription and over-the-counter medicines, vitamins, and herbal products.

Keep all appointments with your doctor and the laboratory. Your doctor will order certain lab tests to check your response to methenamine.

Do not let anyone else take your medication. Ask your pharmacist any questions you have about refilling your prescription.

It is important for you to keep a written list of all of the prescription and nonprescription (over-the-counter) medicines you are taking, as well as any products such as vitamins, minerals, or other dietary supplements. You should bring this list with you each time you visit a doctor or if you are admitted to a hospital. It is also important information to carry with you in case of emergencies.