Hiprex (Methenamine) - Oral

What Is Hiprex?

Hiprex (methenamine) is an orally administered prescription medication used to prevent and control recurrent urinary tract infections (UTIs) in adults and children 6 and older.

Hiprex is an antibiotic that eliminates bacteria caused by UTIs. This medication is often used on a long-term basis.

Antibiotics treat bacterial infections. Specifically, they work by killing bacteria or by making it hard for bacteria to multiply.

In acidic urine, Hiprex works by breaking it down into formaldehyde and ammonia. As such, these substances kill certain bacteria. The drug also makes the urine more acidic than usual. The urine must be acidic for Hiprex to work well.

Hiprex contains the active ingredient methenamine. Methenamine is available as a generic product available via oral administration in the form of tablets to be taken by mouth.

Hiprex, similar to other brand-name medications, such as Urex, is available in the form of oral tablets to be taken by mouth.

Drug Facts

Generic Name: Methenamine

Brand Name(s): Hiprex, Urex

Drug Availability: Prescription

Administration Route: Oral

Therapeutic Classification: Antiseptic

Available Generically: Yes

Controlled Substance: N/A

Active Ingredient: Methenamine hippurate

Dosage Form(s): Tablet

What Is Hiprex Used For?

The Food and Drug Administration (FDA) has approved Hiprex to prevent or control recurrent urinary tract infections (UTIs). Healthcare providers prescribe Hiprex to people needing long-term prevention because their UTIs keep returning. 

For context, a UTI is an infection of the urinary system. The urinary system is made up of the bladder, urethra, kidneys, and ureters. Therefore, UTIs mainly involve the lower urinary tract, specifically the urethra and bladder.

Hiprex should be used only to treat or prevent infections that are proven or suspected to be caused by a bacterial infection.

Moreover, Hiprex should not be used to treat an active UTI. The medication should not be started until after an active infection has been treated with a different antibiotic.

How to Take Hiprex

The typical dosage of Hiprex is 1/2 to 1 tablet twice daily. Take Hiprex by mouth, once in the morning and once at night.

Drink plenty of water while taking Hiprex. It is important to stay hydrated while taking this medication.

This medication works best in acidic urine.

Your healthcare provider will likely recommend taking Hiprex along and a supplement that makes your urine more acidic. Some examples of such supplements are vitamin C (ascorbic acid) and cranberry.

Taking tablets with food may lessen the chance of certain side effects, such as an upset stomach.

Storage

Store Hiprex in a closed container, away from harsh light or moisture. Keep it at room temperature (68 to 77 degrees F), and do not store the tablets in a bathroom.

You can temporarily store Hiprex at lower or higher temperatures ranging from 59 to 86 degrees F, such as when you need to travel or transport the medication.

How Long Does Hiprex Take to Work?

Hiprex starts working right away. A single-gram dose of Hiprex begins having antibacterial effects in the urine 30 minutes after it is taken.

Hiprex provides continuous antibacterial effects when it is taken twice daily. Be sure to take the dosage your provider prescribes.

What Are the Side Effects of Hiprex?

This is not a complete list of side effects, and others may occur. A healthcare provider can advise you on side effects.

If you experience other effects, contact your pharmacist or a medical professional. You may report side effects to the FDA at fda.gov/medwatch or 800-FDA-1088.

Common Side Effects

Possible common side effects of methenamine hippurate are:

Severe Side Effects

Call your healthcare provider right away if you have serious side effects. Call 911 if your symptoms feel life-threatening or if you think you’re having a medical emergency. 

Potential serious side effects associated with the use of Hiprex include the following:

Report Side Effects

Hiprex may cause other side effects. Call your healthcare provider if you have any unusual problems while taking this medication.

If you experience a serious side effect, you or your healthcare provider may send a report to the FDA's MedWatch Adverse Event Reporting Program or by phone (800-332-1088).

Dosage: How Much Hiprex Should I Take?

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The dose of this medicine will be different for different patients. Follow your doctor's orders or the directions on the label. The following information includes only the average doses of this medicine. If your dose is different, do not change it unless your doctor tells you to do so.

The amount of medicine that you take depends on the strength of the medicine. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using the medicine.

  • For the treatment of urinary tract infections:
    • For oral dosage form (methenamine hippurate tablets):
      • Adults and children 12 years of age and over—1 gram two times a day. Take in the morning and the evening.
      • Children up to 6 years of age—Use and dose must be determined by your doctor.
      • Children 6 to 12 years of age—500 milligrams (mg) to 1 gram two times a day. Take in the morning and the evening.
    • For oral dosage form (methenamine mandelate enteric-coated tablets, regular tablets, solution, and suspension):
      • Adults and children 12 years of age and over—1 gram four times a day. Take after meals and at bedtime.
      • Children up to 6 years of age—Dose is based on body weight. The usual dose is 18.3 mg per kilogram (kg) (8.3 mg per pound) of body weight four times a day. Take after meals and at bedtime.
      • Children 6 to 12 years of age—500 mg four times a day. Take after meals and at bedtime.

Modifications

Potential users should note the following before starting treatment with Hiprex:

Dosing adjustments: Hiprex tablets may be cut or split in half. One half-tablet provides a dose of 0.5 grams (which is equal to 500 milligrams).

Pregnancy: In early pregnancy, the safe use of Hiprex has not been confirmed. In the last trimester, safety is suggested but not proven. No negative effects on the fetus were seen in studies in pregnant rats and rabbits. Nonetheless, discuss your pregnancy status with your healthcare provider before starting Hiprex.

Children: Hiprex is suitable for adults and children 6 and older who suffer from recurrent UTIs.

Adults 65 and older: Clinical studies of Hiprex did not include enough people 65 and over to determine whether they respond differently from younger people. However, dose selection for an older person should be made cautiously, starting at the low end of the dosing range to reflect the greater frequency of compromised liver, kidney, or heart function.

Kidney and liver health: Speak to your healthcare professional if you have kidney disease, liver disease, or severe dehydration, as Hiprex is not recommended for people with these conditions.

Missed Dose

Missing doses of Hiprex may make this medication less effective.

Moreover, missed doses may lead to your UTIs returning. Skipping doses can also increase the risk that bacteria could become resistant to methenamine hippurate. This can make it harder to prevent and treat future UTIs.

To avoid missing doses of Hiprex, set an alarm to remind you. Also, take the medication with you when you are away from home at your usual dose time.

Overdose: What Happens If I Take Too Much Hiprex?

While generally safe and well-tolerated, taking too much Hiprex may cause bladder irritation, dysuria (painful urination), frequent urination, or proteinuria (excessive amounts of protein in the urine).

What Happens If I Overdose on Hiprex?

If you think you or someone else may have overdosed on Hiprex, call a healthcare provider or the Poison Control Center (800-222-1222).

If someone collapses or isn't breathing after taking Hiprex, call 911 immediately.

Precautions


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If your symptoms do not improve within a few days, or if they become worse, check with your doctor.

What Are Reasons I Shouldn’t Take Hiprex?

Hiprex is not for everyone.

These tablets should only be prescribed to people who need preventative treatment for long-term UTIs that keep coming back.

Healthcare providers may not prescribe methenamine hippurate to people with the following medical conditions:

  • Kidney problems
  • Severe liver problems
  • Serious dehydration

Additionally, people who have had a past allergic reaction to methenamine should not take it.

What Other Medications Interact With Hiprex?

Before taking Hiprex, tell your healthcare provider about all your medications, including prescription and over-the-counter (OTC) medicines, and vitamins or supplements.

Your healthcare provider can advise you on any possible drug interactions. The list below includes some of the drugs that interact with methenamine.

Sulfonamide medications interact with methenamine. The combination is not recommended because the two drugs can bind together to form crystals in the urine.

Examples of sulfonamides include:

Medications or supplements that make the urine less acidic can make Hiprex less effective.

Thus, if possible, avoid the following medicines while taking Hiprex:

What Medications Are Similar?

The list below describes drugs that are alternatives to Hiprex. It is not a list of drugs recommended to take with Hiprex. You should not take these drugs together. 

Like Hiprex, the following prescription medications are used to prevent UTIs:

  • Uribel is a combination drug that contains several active ingredients. Besides methenamine, it contains sodium phosphate (to make the urine more acidic), phenyl salicylate (a pain reliever similar to aspirin), methylene blue (an antiseptic), and Levsin (hyoscyamine) (an antispasmodic drug).
  • Methenamine mandelate (formerly known as Mandelamine). This medication is similar to methenamine hippurate, but typically requires more frequent dosing (four times per day).
  • Furadantin (nitrofurantoin). This is a low-dose antibiotic that can be used to prevent UTIs. 

Ask your healthcare provider if you have questions about alternatives to Hiprex.

Frequently Asked Questions

  • My healthcare provider says that Hiprex works best in acidic urine. How can I tell if my urine is acidic?

    Yes, Hiprex works best when your urine is acidic, and acids have a low pH (pH refers to whether a substance is an acid, an alkaline (a basic), or a neutral.

    You can check your urine pH at home using urine pH test strips. You can order pH test strips online or purchase them from a pharmacy.

  • Should I avoid any OTC medicines while taking Hiprex?

    Yes. Certain OTC medicines and supplements can make your urine less acidic. Hiprex works best when your urine is more acidic.

    Avoid antacids like Gaviscon and Mylanta. Talk to your healthcare provider or pharmacist to find a suitable alternative to antacids.

  • What are the common side effects of methenamine hippurate?

    Not everyone who takes Hiprex experiences side effects. However, it’s possible to experience nausea, vomiting, upset stomach, rash, itching, or painful urination as a result of using the medication.

    Painful urination can also be a symptom of a UTI. Call your healthcare provider if you develop other UTI symptoms while taking Hiprex.

    They can advise whether you should stop taking Hiprex until you receive treatment for the UTI if needed.

  • Should I stop taking methenamine hippurate if I start having UTI symptoms?

    Hiprex isn’t meant to treat an active UTI.

    If you have symptoms that you recognize as signs of a UTI, call your healthcare provider. They’ll likely recommend stopping Hiprex until after your UTI is treated.

How Can I Stay Healthy While Taking Hiprex?

Taking Hiprex as directed by your healthcare provider can be an effective preventative treatment for people with recurrent or chronic (long-term) UTIs. Hiprex works best in acidic urine.

To ensure you get the most benefit possible from this medicine, follow your healthcare provider’s advice for making your urine more acidic.

This may include taking methenamine with a supplement, such as vitamin C or cranberry. It’s also important not to skip doses. Do not stop taking this medication unless your healthcare provider advises it.

Taking Hiprex is just one way to prevent UTIs. Be sure to stay hydrated by drinking plenty of water while taking Hiprex. Learn more about other ways to support bladder health, including tips for keeping your urinary system healthy.

Medical Disclaimer

Verywell Health's drug information is meant for educational purposes only and is not intended to replace medical advice, diagnosis, or treatment from a healthcare provider. Consult your healthcare provider before taking any new medication(s). IBM Watson Micromedex provides some of the drug content, as indicated on the page.

13 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Patricia Weiser, PharmD
Patricia Weiser, PharmD, is a licensed pharmacist and freelance medical writer. She has more than 14 years of professional experience.