Methenamine

You should not use methenamine if you have kidney disease, severe liver disease, if you also take a sulfa drug, or if you are dehydrated.

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

• kidney disease;

• severe liver disease;

• if you also take a sulfa drug (Azulfidine, Bactrim, Septra, SMZ-TMP, Sulfazine, and others); or

• if you are dehydrated.

To make sure methenamine is safe for you, tell your doctor if you have:

• liver disease;

• asthma;

• an allergy to aspirin; or

• if you are allergic to yellow food dye.

FDA pregnancy category C. It is not known whether methenamine will harm an unborn baby. Tell your doctor if you are pregnant or plan to become pregnant while using this medicine.

Methenamine can pass into breast milk and may harm a nursing baby. Tell your doctor if you are breast-feeding a baby.

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

Specific Drugs and Laboratory Tests

Storage

Oral

15–30°C100 101 102 in tight, light resistant containers.102

Excipients in commercially available drug preparations may have clinically important effects in some individuals; consult specific product labeling for details.

Please refer to the ASHP Drug Shortages Resource Center for information on shortages of one or more of these preparations.

* available from one or more manufacturer, distributor, and/or repackager by generic (nonproprietary) name

Along with its needed effects, a medicine 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:

• Skin rash

• Blood in urine
• lower back pain
• pain or burning while urinating

Some side effects 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:

• Nausea and vomiting

Other side effects not listed may also occur in some patients. If you notice any other effects, check with your healthcare professional.

Call your doctor for medical advice about side effects. You may report side effects to the FDA at 1-800-FDA-1088.

• If you have an allergy to methenamine or any other part of methenamine.
• If you are allergic to any drugs like this one, any other drugs, foods, or other substances. Tell your doctor about the allergy and what signs you had, like rash; hives; itching; shortness of breath; wheezing; cough; swelling of face, lips, tongue, or throat; or any other signs.
• If you have any of these health problems: Fluid loss (dehydrated), kidney disease or liver disease.
• If you are taking a sulfa (sulfonamide) drug. Ask your doctor if you are not sure if any of your drugs are a sulfa drug.

This is not a list of all drugs or health problems that interact with this medicine.

Tell your doctor and pharmacist about all of your drugs (prescription or OTC, natural products, vitamins) and health problems. You must check to make sure that it is safe for you to take methenamine with all of your drugs and health problems. Do not start, stop, or change the dose of any drug without checking with your doctor.

If you think there has been an overdose, call your poison control center or get medical care right away. Be ready to tell or show what was taken, how much, and when it happened.

• If your symptoms or health problems do not get better or if they become worse, call your doctor.
• Do not share your drugs with others and do not take anyone else's drugs.
• Keep a list of all your drugs (prescription, natural products, vitamins, OTC) with you. Give this list to your doctor.
• Talk with the doctor before starting any new drug, including prescription or OTC, natural products, or vitamins.
• Some drugs may have another patient information leaflet. Check with your pharmacist. If you have any questions about methenamine, please talk with your doctor, nurse, pharmacist, or other health care provider.
• If you think there has been an overdose, call your poison control center or get medical care right away. Be ready to tell or show what was taken, how much, and when it happened.

This information should not be used to decide whether or not to take this medicine or any other medicine. Only the healthcare provider has the knowledge and training to decide which medicines are right for a specific patient. This information does not endorse any medicine as safe, effective, or approved for treating any patient or health condition. This is only a brief summary of general information about methenamine. It does NOT include all information about the possible uses, directions, warnings, precautions, interactions, adverse effects, or risks that may apply to this medicine. This information is not specific medical advice and does not replace information you receive from the healthcare provider. You must talk with the healthcare provider for complete information about the risks and benefits of using methenamine.

Review Date: October 4, 2017

Prescribing Methenamine hippurate tablets, USP in the absence of a proven or strongly suspected bacterial infection or a prophylactic indication is unlikely to provide benefit to the patient and increases the risk of the development of drug-resistant bacteria.

1.  Care should be taken to maintain an acid pH of the urine, especially when treating infections due to urea-splitting organisms such as Proteus and strains of Pseudomonas.

2.  In a few instances in one study, the serum transaminase levels were slightly elevated during treatment but returned to normal while the patients were still taking Methenamine hippurate tablets, USP. Because of this report, it is recommended that liver function studies be performed periodically on patients taking the drug, especially those with liver dysfunction.

3.  Use in Pregnancy: In early pregnancy the safe use of Methenamine hippurate tablets, USP is not established. In the last trimester, safety is suggested, but not definitely proved. No adverse effects on the fetus were seen in studies in pregnant rats and rabbits.
    
Methenamine hippurate tablets, USP taken during pregnancy can interfere with laboratory tests of urine estriol (resulting in unmeasurably low values) when acid hydrolysis is used in the laboratory procedure. This interference is due to the presence in the urine of Methenamine and/or formaldehyde. Enzymatic hydrolysis, in place of acid hydrolysis, will circumvent this problem.

Information for Patients

Patients should be counseled that antibacterial drugs including Methenamine hippurate tablets, USP should only be used to treat bacterial infections. They do not treat viral infections (e.g., the common cold). When Methenamine hippurate tablets, USP are prescribed to treat a bacterial infection, patients should be told that although it is common to feel better early in the course of therapy, the medication should be taken exactly as directed. Skipping doses or not completing the full course of therapy may (1) decrease the effectiveness of the immediate treatment and (2) increase the likelihood that bacteria will develop resistance and will not be treatable by Methenamine hippurate tablets,  USP or other antibacterial drugs in the future.

Geriatric Use

Clinical studies of Methenamine hippurate tablets, USP did not include sufficient numbers of subjects aged 65 and over to determine whether they respond differently from younger subjects. Other reported clinical experience has not identified differences in responses between the elderly and younger patients. In general, dose selection for an elderly patient should be cautious, usually starting at the low end of the dosing range, reflecting the greater frequency of decreased hepatic, renal or cardiac function, and of concomitant disease or other drug therapy.

Methenamine hippurate tablets, USP are contraindicated in patients with renal insufficiency and severe hepatic insufficiency (see CONTRAINDICATIONS).

Hypersensitivity to methenamine or any component of the formulation; severe dehydration; renal impairment; severe hepatic impairment; concurrent treatment with sulfonamides

Adverse events have not been observed in animal reproduction studies with methenamine hippurate; animal reproduction studies have not been conducted with methenamine mandelate. Methenamine crosses the placenta and distributes to amniotic fluid (Allgén, 1979). An increased risk of adverse fetal effects has not been observed in available studies (Furness, 1975; Gordon, 1972; Heinonen, 1977). Methenamine use has been shown to interfere with urine estriol concentrations if measured via acid hydrolysis. Use of enzyme hydrolysis prevents this lab interference.