Hiprex

Introduction

Written by Sarah Lewis, PharmD
Medically reviewed by Dena Westphalen, PharmD 
Updated on October 9, 2023

Methenamine is a generic medication prescribed to prevent urinary tract infections (UTIs) that keep coming back. Methenamine belongs to a group of drugs called urinary antiseptics.

Side effects of methenamine

Methenamine may cause mild or serious side effects. More common mild side effects of methenamine and its serious side effects are listed below. This article doesn’t include all possible side effects of the drug. Side effects can vary based on your age, overall health, and any other medications you take.

To learn more about methenamine’s side effects, ask your doctor or pharmacist. You can also refer to the prescribing information for methenamine listed in the “Resources” section.

Mild side effects

More common mild side effects reported with some forms of methenamine are listed below. 

This article doesn’t include all possible mild side effects of the drug. To learn more about methenamine’s side effects, refer to the prescribing information for methenamine listed in the “Resources” section.

With many drugs, mild side effects may go away within a few days or a couple of weeks. If the side effects are bothersome, tell your doctor or pharmacist. They may be able to suggest ways to manage them.

Methenamine oral tablet’s mild side effects include:

• nausea
• upset stomach
• mild allergic reaction*

* For details about this side effect, see the “Interactions and warnings for methenamine” section.

Serious side effects

Serious side effects of methenamine are listed below. 

This article doesn’t include all possible serious side effects of the drug. To learn more about methenamine’s side effects, refer to the prescribing information for methenamine listed in the “Resources” section.

With many drugs, serious side effects are possible but not common. If you have serious side effects from this drug, call your doctor right away. If you’re having severe symptoms or a medical emergency, call 911 or a local emergency number. 

In general, methenamine’s serious side effects include:

• albumin (a blood protein) in the urine, which can be a sign of kidney problems
• bladder irritation
• blood in urine
• frequent and painful urination
• severe allergic reaction*

* For details about this side effect, see the “Interactions and warnings for methenamine” section.

Uses of methenamine

The Food and Drug Administration (FDA) approves prescription drugs such as methenamine for certain conditions. Approved uses for methenamine are described below. 

Use for recurrent urinary tract infections

Doctors may prescribe methenamine oral tablet for helping to prevent recurrent urinary tract infections (UTIs). It’s used for this purpose in adults and children 6 years and older. It’s usually prescribed as a long-term treatment.

With recurrent UTIs, the infections keep coming back. If this happens, doctors may suggest long-term treatment to suppress them. Before using methenamine oral tablet, doctors prescribe an antibiotic to clear the infection. Then, methenamine oral tablet can help keep the infection from happening again.

Interactions and warnings for methenamine

Below you’ll find information about methenamine’s possible interactions and warnings. 

Interactions of methenamine

Certain things may affect how a drug works. These include getting vaccines, consuming alcohol or certain foods, or taking the drug with other medications. This effect is called a drug interaction. 

Before you take methenamine, ask your doctor to check for possible interactions. Be sure to tell them about any of the following you take or use:

• prescription or over-the-counter drugs
• vitamins, herbs, or supplements

To learn about drug-condition interactions, see the “Warnings for methenamine” section below.

Warnings for methenamine

Some people should not take methenamine, and others should take it cautiously.

Methenamine could cause harm to people with certain health conditions. This effect is called a drug-condition interaction. Other factors can also affect whether methenamine is a safe option for you. 

Ask your doctor about specific warnings for methenamine, and be sure to tell your doctor about your:

• current health, including any allergies to medications
• past health conditions or surgeries

Allergic reaction

Methenamine can cause an allergic reaction in some people. Severe allergic reactions are rare but possible.

If you’ve had an allergic reaction to methenamine or any of its ingredients, your doctor will likely not prescribe methenamine. They can tell you about medications that are safer options for you.

A mild allergic reaction may cause the following symptoms:

• skin rash
• itchiness
• flushing (temporary warmth, redness, or deepening of skin color)

A severe allergic reaction may cause the following symptoms:

• swelling under your skin, usually in your hands, feet, eyelids, or lips
• swelling of your mouth, throat, or tongue, which can cause breathing problems 

If you have an allergic reaction to methenamine, call your doctor right away. If you have severe symptoms, call 911 or a local emergency number.

Overdose of methenamine

If you take methenamine by mouth, you should not take more methenamine than your doctor prescribes. For some drugs, doing so may lead to harmful effects or overdose.

If you think you’ve taken too much methenamine, call your doctor or pharmacist right away. Or you could call 800-222-1222 to speak with someone at America’s Poison Centers. You can also use its online resource. If you have concerning symptoms, call 911 or a local emergency number immediately. You can also go to the closest emergency room.

Pregnancy or breastfeeding and methenamine

Information about methenamine and pregnancy and breastfeeding is described below.

Methenamine and pregnancy

It’s not known whether methenamine should be taken during pregnancy. If you’re planning a pregnancy or can become pregnant, talk with your doctor before taking this medication. 

Methenamine and breastfeeding

It’s not known whether methenamine should be taken while breastfeeding. If you’re currently breastfeeding or planning to do so, talk with your doctor before taking this medication. 

Common questions about methenamine

Below you’ll find answers to a few commonly asked questions about methenamine.

Is methenamine safe for older adults?

Possibly. In studies, there wasn’t enough evidence to show whether adults 65 years and older respond differently than younger adults to methenamine. But older adults can have problems with kidney function that happen with age. Methenamine oral tablet shouldn’t be used in people with poor kidney function due to problems with blood flow to the kidneys.

If you have concerns about taking methenamine oral tablet due to your age, talk with your doctor.

How does methenamine work? And how long does it take to start working?

Methenamine oral tablet works by acting as an antiseptic. Antiseptics clean and disinfect an area, but they don’t treat infections. Methenamine oral tablet will start working to clean the urine within 30 minutes of taking it. 

If you’d like to know more about how methenamine works, talk with your doctor or pharmacist. 

What to ask your doctor

Let your doctor know if you have questions about methenamine or would like more details about it.

Here’s a list of some possible questions you may want to ask your doctor:

• What time of day should I take methenamine oral tablet?
• What foods should I eat to help methenamine oral tablet work best?
• Do any of my other medications interact with methenamine oral tablet?

Resources

• Food and Drug Administration. (2023). Orange Book: Approved drug products with therapeutic equivalence evaluations. https://www.accessdata.fda.gov/scripts/cder/ob/index.cfm
• Methenamine hippurate tablets, USP. (2023). https://nctr-crs.fda.gov/fdalabel/services/spl/set-ids/4412d667-63f6-427e-a5c6-376a0e1e172d/spl-doc 

Disclaimer: Optum Perks has made every effort to make certain that all information is factually correct, comprehensive, and up to date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or another healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.