Uses

What is methenamine used for?

Methenamine is commonly used to prevent or reduce the chances of getting frequent kidney or bladder infections (urinary tract infections) that are caused by certain bacteria, especially when you need treatment for a long time.

Methenamine may not be appropriate for some of the infections listed above, depending on which germs are causing the infection. Bacteria in some locations may have developed resistance to methenamine, which will make it not work as well.

Methenamine may also be used for other conditions as determined by your healthcare provider.

How does methenamine work (mechanism of action)?

Methenamine works by making your urine (pee) more acidic, which helps stop bacteria from growing in your urinary tract. This helps prevent future infections.

How is methenamine supplied (dosage forms)?

Methenamine is available as Hiprex and generic methenamine in the following dosage forms that are taken by mouth.

  • 500 mg oral tablets 
  • 1 g oral tablets

How should I store methenamine?

Methenamine should be stored at room temperature, between 68 F to 77 F (20 C to 25 C). It can be exposed to temperatures between 59 F to 86 F (15 C to 30 C) for shorter periods of time, such as when transporting it. Store in a cool, dry place.

Side Effects

What are the most common side effects of methenamine?

The most common side effects of methenamine are listed below. Tell your healthcare provider if you have any of these side effects that bother you.

  • Nausea or upset stomach
  • Rash
  • Problems peeing (see below)

There may be other side effects of methenamine that are not listed here. Contact your healthcare provider if you think you are having a side effect of a medicine. In the U.S., you can report side effects to the FDA at www.fda.gov/medwatch or by calling 800-FDA-1088. In Canada, you can report side effects to Health Canada at www.health.gc.ca/medeffect or by calling 866-234-2345.

What are the serious side effects of methenamine?

While less common, the most serious side effects of methenamine are described below, along with what to do if they happen.

Severe Allergic Reactions. Methenamine may cause allergic reactions, which can be serious. Stop taking methenamine and get help right away if you have any of the following symptoms of a serious allergic reaction.

  • Breathing problems or wheezing
  • Racing heart
  • Fever or general ill feeling
  • Swollen lymph nodes
  • Swelling of the face, lips, mouth, tongue, or throat
  • Trouble swallowing or throat tightness
  • Itching, skin rash, or pale red bumps on the skin called hives
  • Nausea or vomiting
  • Dizziness, feeling lightheaded, or fainting
  • Stomach cramps
  • Joint pain

Problems Peeing. Taking methenamine, especially higher doses, can cause pain when peeing or needing to pee often. Tell your healthcare provider right away if you develop stinging, burning, or itching sensation when you start to pee or after you pee. 

Blood in Urine (Hematuria). Methenamine may cause blood in your urine. This can be serious. Call your healthcare provider right away if you have any of the following symptoms.

  • Pink, red, or brown pee
  • Burning or pain when you pee
  • A strong urge to pee
  • Pain in your lower belly, lower back, pelvis, or side
  • Fever or chills
  • Nausea or vomiting
  • Loss of appetite 
  • Weakness or unusual tiredness 

Protein in Urine (Albuminuria). Methenamine may cause albumin, a protein that is normally found in blood, to show up in your urine. This can be a sign your kidneys are not functioning properly. Call your healthcare provider right away if you have any of the following symptoms of kidney problems.

  • Reduced need to pee
  • Swelling in your feet, ankles, or legs
  • Weakness or unusual tiredness
  • Difficulty catching your breath or chest pain/pressure
  • Confusion
  • Nausea
  • Seizures

Kidney Stones. Methenamine may cause uric acid crystals to form in your urine, which can lead to kidney stones. Drinking plenty of water can help prevent kidney stones. Tell your healthcare provider right away if you develop severe pain in your side or back or have blood in your urine (pee). Your pee may look pink, red, or brown.

Warnings & Precautions

Who should not use methenamine?

Allergies to Ingredients. People who are allergic to any of the following should not take methenamine.

  • Methenamine
  • Hiprex
  • Urex
  • Any of the ingredients in the specific product dispensed

Your pharmacist can tell you all of the ingredients in the specific methenamine products they stock.

Other Medicines and Supplements. Methenamine may interact with other medicines and supplements. Before taking methenamine, tell your healthcare provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. See the Interactions section for more details.

Kidney Problems. Methenamine should not be used if your kidneys are not working as well as they should be. If there is a concern about the health of your kidneys, your healthcare provider may do tests to determine if they are working well enough to take this medicine.

Liver Problems. Methenamine should not be used if your liver is not working as well as it should be. If there is a concern about the health of your liver, your healthcare provider may do tests to determine if it is working well enough to take this medicine.

Dehydration. Methenamine should not be used if you have severe dehydration. Dehydration may lead to low blood pressure, especially when you stand up (orthostatic hypotension), or kidney damage. Call your healthcare provider right away if you have any of the following symptoms of dehydration, low blood pressure, or kidney damage.

  • Dizziness or lightheadedness
  • Weakness or unusual tiredness
  • Confusion
  • Unsteadiness
  • Peeing less than normal
  • Swelling in your legs, ankles, and feet
  • Loss of appetite or nausea
  • Difficulty catching your breath or chest pain or pressure
  • Seizures
  • Extreme thirst

What should I know about methenamine before using it?

Do not take methenamine unless it has been prescribed to you by a healthcare provider. Take it as prescribed.

Do not share methenamine with other people, even if they have the same condition as you. It may harm them.

Keep methenamine out of the reach of children.

Your healthcare provider may ask you to test how acidic your urine is at home while taking methenamine. Talk with your healthcare provider or pharmacist to ensure you are using the right product. 

It is important to keep all of your healthcare appointments while you are taking methenamine, including lab appointments. These lab tests help your healthcare provider know if the medicine is working. 

Tell all of your healthcare providers that you are taking methenamine. Methenamine can affect the results of certain laboratory tests.  

People who are 65 and older can be at greater risk for some side effects from methenamine. Talk to your healthcare provider about your risks if you are in this age group.

Some methenamine products may contain an ingredient called FD&C Yellow No. 5, also called tartrazine. If you have an allergy to aspirin, you may be more likely to have a tartrazine allergy.

What should I tell my healthcare provider before using methenamine?

Tell your healthcare provider about all of your health conditions and any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. This will help them determine if methenamine is right for you.

In particular, make sure that you discuss any of the following.

Current and Past Health Conditions. Tell your healthcare provider if you have any of the following.

  • Kidney stones
  • Gout

Pregnancy. It is not known if or how methenamine could affect pregnancy or harm an unborn baby. Tell your healthcare provider if you are or plan to become pregnant. Your healthcare provider will advise you if you should take methenamine while you are pregnant or trying to get pregnant. 

Breastfeeding. Methenamine passes into breast milk. Tell your healthcare provider if you are breastfeeding or plan to breastfeed. Your healthcare provider will advise you if you should stop breastfeeding or stop methenamine.

Interactions

Does methenamine interact with foods or drinks?

Avoid drinking milk or using other milk products while taking methenamine. These can lessen how acidic the urine is, which can affect how well the medicine works.

It is unknown if drinking alcohol will affect methenamine, but heavy drinking can slow your immune response and make it harder for your body to fight infection.

Does methenamine interact with other medicines (drug interactions)?

Always tell your healthcare provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using.

In particular, make sure that you discuss if you are using any of the following before taking methenamine.

  • A sulfa or sulfonamide medicine, such as sulfamethoxazole, sulfadiazine silver, or sulfasalazine
  • Aspirin, which is in many prescription and OTC products, including those for pain, swelling, and fever
  • An antacid that contains sodium carbonate or bicarbonate

 This may not be a complete list of medicines that can interact with methenamine. Always check with your healthcare provider.

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Overdose/Missed Dose

What should I do if I accidentally use too much methenamine?

If you or someone else has used too much methenamine, get medical help right away, call 911, or contact a Poison Control center at 800-222-1222.

What should I do if I miss a dose of methenamine?

If you miss a dose, take it as soon as you remember. If it is almost time for your next dose, skip the missed dose and only take the next dose. Do not take double or extra doses.

It is important to complete your prescribed course of methenamine, even if your symptoms get better or go away. This will reduce the risk that future infections are resistant to methenamine.

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