What is methenamine?

Methenamine is a urinary antiinfective medicine. Methenamine fights bacteria in the urine and bladder.

Methenamine may be used to treat and prevent urinary tract infections.

Methenamine may also be used for purposes other than those listed here.


Before taking methenamine, talk to your doctor if you

  • have liver problems;
  • have kidney problems;
  • have other medical conditions; or
  • take other medications.

You may not be able to take methenamine, or you may require a dosage adjustment or special monitoring during treatment.

Methenamine is in the FDA pregnancy category C. This means that it is not known whether it will be harmful to an unborn baby. Do not take methenamine without first talking to your doctor if you are pregnant or could become pregnant during treatment.

Methenamine passes into breast milk and may affect a nursing baby. Do not take methenamine without first talking to your doctor if you are breast-feeding a baby.

Your healthcare provider may recommend drinking additional water and/or certain fruit juices (e.g., cranberry, plum, prune) and increased protein in the diet while taking methenamine to ensure adequate hydration and acidity of the urine. It may also be recommended to avoid citrus fruits and juices (e.g., orange, grapefruit, lemon), milk and dairy products, and antacids during treatment with methenamine. These products may decrease the effectiveness of methenamine. Follow your healthcare provider’s instructions.

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Seek emergency medical attention if an overdose is suspected.

Symptoms of a methenamine overdose are not known.

Take the missed dose as soon as you remember. However, if it is almost time for the next dose, skip the dose you missed and take only the next regularly scheduled dose. Do not take a double dose of this medication unless otherwise directed by your doctor.

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Side effects

If you experience any of the following serious side effects, stop taking methenamine and seek emergency medical attention or contact your doctor immediately:

  • an allergic reaction (shortness of breath; closing of the throat; swelling of the lips, face, or tongue; or hives;
  • lower back or side pain;
  • blood in urine; or
  • increasingly painful or difficult urination.

Other, less serious side effects may be more likely to occur. Continue to take methenamine and talk to your doctor if you experience

  • nausea or upset stomach;
  • decreased appetite; or
  • skin rash.

Side effects other than those listed here may also occur. Talk to your doctor about any side effect that seems unusual or that is especially bothersome. You may report side effects to FDA at 1-800-FDA-1088.

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Do not take methenamine without first talking to your doctor if you are taking:

  • a carbonic anhydrase inhibitor such as acetazolamide (Diamox), dichlorphenamide (Daranide), or methazolamide (Glauctabs, MZM, Neptazane);
  • a sulfa product such as sulfadiazine, sulfamethoxazole (Bactrim, Septra, others), sulfasalazine (Azulfidine), and others;
  • a diuretic (water pill); or
  • a product that contains aluminum, calcium, magnesium, sodium bicarbonate, potassium or sodium citrate, or citric acid (such as antacids, vitamin or mineral pills, urinary alkalinizers, and other medications).

You may not be able to take methenamine, or you may require a dosage adjustment or special monitoring during treatment if you are taking any of the medicines listed above.

Drugs other than those listed here may also interact with methenamine. Talk to your doctor and pharmacist before taking any prescription or over-the-counter medicines, including vitamins, minerals, and herbal products.

Other Names

Hiprex, Mandelamine, Urex, and methenamine

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Every effort has been made to ensure that the information provided by Cerner Multum, Inc. ('Multum') is accurate, up-to-date, and complete, but no guarantee is made to that effect. Drug information contained herein may be time sensitive. Multum information has been compiled for use by healthcare practitioners and consumers in the United States and therefore Multum does not warrant that uses outside of the United States are appropriate, unless specifically indicated otherwise. Multum's drug information does not endorse drugs, diagnose patients or recommend therapy. Multum's drug information is an informational resource designed to assist licensed healthcare practitioners in caring for their patients and/or to serve consumers viewing this service as a supplement to, and not a substitute for, the expertise, skill, knowledge and judgment of healthcare practitioners. The absence of a warning for a given drug or drug combination in no way should be construed to indicate that the drug or drug combination is safe, effective or appropriate for any given patient. Multum does not assume any responsibility for any aspect of healthcare administered with the aid of information Multum provides. The information contained herein is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. If you have questions about the drugs you are taking, check with your doctor, nurse or pharmacist.

Copyright 1996-2004 Cerner Multum, Inc. Version 2.05. Revision date 8/23/04

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