Hiprex: Uses, Dosage & Side Effects
A urinary antiseptic containing methenamine hippurate used to prevent recurrent urinary tract infections and reduce infection risk during urological procedures and long-term catheterization
Hiprex (methenamine hippurate) is a urinary antiseptic used to prevent recurrent urinary tract infections (UTIs) in individuals who are prone to repeated episodes. Unlike conventional antibiotics, Hiprex works by releasing formaldehyde in acidic urine, which kills bacteria in the urinary tract without promoting antimicrobial resistance. It is also used prophylactically during urological investigations and procedures, and in patients with long-term urinary catheters. Hiprex is available as a 1 g tablet and requires a prescription. It represents an important non-antibiotic strategy in the growing global effort to combat antimicrobial resistance.
Quick Facts: Hiprex
Key Takeaways
- Hiprex (methenamine hippurate) is a urinary antiseptic that prevents recurrent urinary tract infections by releasing bactericidal formaldehyde in acidic urine, offering a non-antibiotic alternative that does not promote antimicrobial resistance.
- The standard adult dose is 1 g taken orally two to three times daily; children over 12 years take 1 g twice daily. The tablet should be swallowed with water, and adequate fluid intake is recommended throughout treatment.
- Hiprex must not be used in patients with renal impairment, severe dehydration, gout, hepatic impairment, or metabolic acidosis, and must never be combined with sulfonamide antibiotics due to the risk of crystalluria.
- Common side effects include nausea, vomiting, skin rash, and bladder irritation; these are generally mild and reversible. Serious adverse effects are rare when the drug is used as directed.
- A 2024 Cochrane systematic review confirmed the efficacy of methenamine hippurate in preventing UTIs in patients without renal tract abnormalities, supporting its role as a first-line non-antibiotic prophylactic option recommended by international guidelines including the European Association of Urology (EAU).
What Is Hiprex and What Is It Used For?
Hiprex contains the active substance methenamine hippurate, a chemical compound that has been used in clinical medicine for over a century as a urinary antiseptic. Unlike conventional antibiotics that target specific bacterial mechanisms – such as cell wall synthesis or protein production – methenamine works through a fundamentally different mechanism. When methenamine reaches the bladder and encounters acidic urine (pH below 5.5), it undergoes hydrolysis to release formaldehyde, a potent broad-spectrum bactericidal agent. Formaldehyde denatures bacterial proteins and nucleic acids non-specifically, making it effective against virtually all common urinary pathogens.
The hippuric acid component of methenamine hippurate serves a dual purpose. First, hippuric acid itself possesses weak antibacterial properties that contribute to the overall antimicrobial effect. Second, and more importantly, hippuric acid helps acidify the urine, creating a more favorable environment for the hydrolysis of methenamine to formaldehyde. This self-acidifying property distinguishes methenamine hippurate from methenamine mandelate, which may require additional urinary acidification measures.
Hiprex is primarily used in the following clinical scenarios:
- Prevention of recurrent urinary tract infections: Hiprex is indicated for long-term prophylaxis in individuals who are prone to repeated UTIs. Recurrent UTIs are defined as two or more infections within six months, or three or more within twelve months. This is the most common indication, particularly in women who experience frequent cystitis episodes. The European Association of Urology (EAU) 2024 guidelines recommend methenamine hippurate as a non-antibiotic prophylactic option for women with recurrent uncomplicated UTIs.
- Prophylaxis during urological procedures: Hiprex is used before and after urological investigations and treatments (such as cystoscopy, urodynamic studies, or catheter insertions) to reduce the risk of procedure-related urinary tract infections. Bacteriuria following urological instrumentation is common, and prophylactic use of methenamine hippurate can help minimize this risk.
- Long-term catheterization: Patients who require an indwelling urinary catheter for extended periods are at high risk of catheter-associated urinary tract infections (CAUTIs). Hiprex is used in these patients to help maintain urinary antisepsis and reduce the frequency of symptomatic infections.
The growing global concern about antimicrobial resistance has renewed interest in methenamine hippurate as a non-antibiotic strategy for UTI prevention. Because formaldehyde kills bacteria through non-specific protein denaturation rather than targeting a particular bacterial pathway, resistance to methenamine does not develop in the way it does with conventional antibiotics. This makes Hiprex particularly valuable in an era where antibiotic-resistant uropathogens, including extended-spectrum beta-lactamase (ESBL)-producing Escherichia coli, are becoming increasingly prevalent.
The landmark ALTAR trial (Alternative to Prophylactic Antibiotics for the Treatment of Recurrent Urinary Tract Infections in Women), published in the BMJ in 2022, was a large randomized controlled trial that demonstrated methenamine hippurate was non-inferior to daily low-dose antibiotics for preventing recurrent UTIs over a 12-month period. This trial provided high-quality evidence supporting the use of methenamine hippurate as a first-line alternative to long-term antibiotic prophylaxis.
Hiprex is one of the few evidence-based non-antibiotic options for preventing recurrent urinary tract infections. Its unique mechanism of action – releasing formaldehyde in acidic urine – means that bacteria do not develop resistance the way they do with conventional antibiotics. This makes it an important tool in the global effort to reduce unnecessary antibiotic use and combat antimicrobial resistance.
What Should You Know Before Taking Hiprex?
Contraindications
There are specific medical conditions and situations in which Hiprex must not be used. Understanding these absolute contraindications is essential for safe use of this medication.
- Hypersensitivity: Do not take Hiprex if you are allergic to methenamine hippurate or any other ingredient in the product.
- Renal impairment: Hiprex is contraindicated in patients with impaired kidney function. Since methenamine is hydrolyzed to formaldehyde in the urinary tract, impaired renal clearance could lead to systemic accumulation of formaldehyde, which is toxic. The kidneys must be functioning well enough to adequately excrete methenamine and its metabolites.
- Severe dehydration: Dehydration concentrates the urine and may lead to excessive formaldehyde levels in the bladder, causing mucosal irritation. Adequate hydration is essential during Hiprex therapy.
- Gout: Methenamine hippurate is contraindicated in patients with gout. The hippuric acid component may interfere with uric acid metabolism, potentially exacerbating hyperuricemia and triggering gout flares.
- Hepatic impairment: Hiprex should not be used in patients with liver disease, as the metabolism and handling of the drug’s components may be impaired.
- Metabolic acidosis: Patients with pre-existing metabolic acidosis should not take Hiprex, as the hippuric acid component may worsen the acid-base imbalance.
Hiprex must never be taken at the same time as sulfonamide antibiotics (also known as “sulfa drugs”). The combination of formaldehyde (released from methenamine) and sulfonamides can cause crystalluria – the formation of insoluble crystals in the urine – which can damage the kidneys and urinary tract. This is an absolute contraindication.
Warnings and Precautions
Before and during treatment with Hiprex, certain precautions should be observed to ensure the medication works effectively and safely:
- Urinary pH monitoring: Hiprex is only effective when the urine is acidic (pH below 5.5). Conditions, diets, or medications that alkalinize the urine will reduce or eliminate the drug’s antibacterial activity. Patients taking urinary alkalinizers such as potassium citrate mixtures, sodium bicarbonate, or certain antacids should not use Hiprex concurrently.
- Adequate fluid intake: While adequate hydration is important to maintain healthy urine flow, excessive fluid intake may dilute the formaldehyde concentration in the urine below effective bactericidal levels. A moderate, balanced fluid intake is recommended.
- Long-term use: Although Hiprex is intended for prophylactic use over extended periods, regular medical review is advisable. Your doctor may periodically assess kidney function and the ongoing need for prophylaxis.
- Laboratory test interference: Methenamine may cause falsely elevated results for certain urinary laboratory tests, including assays for steroids and catecholamines. If you are undergoing urine testing, inform the laboratory staff that you are taking Hiprex so that results can be interpreted correctly.
- Proteus infections: Bacteria of the genus Proteus produce urease, an enzyme that splits urea into ammonia, raising urinary pH. This alkalinization of the urine reduces the effectiveness of methenamine. If Proteus species are identified as the causative pathogen, alternative treatment strategies may be required.
Pregnancy and Breastfeeding
Methenamine hippurate has a long history of use during pregnancy, and no adverse effects on the fetus have been demonstrated in clinical experience or available studies. However, as with all medications during pregnancy, you should consult your doctor before starting Hiprex to ensure the benefits outweigh any potential risks in your individual situation.
Methenamine hippurate passes into breast milk, but at the recommended therapeutic doses, it is unlikely to affect the breastfed infant. The European Medicines Agency (EMA) considers methenamine hippurate compatible with breastfeeding for short-term use. However, if you require long-term prophylaxis while breastfeeding, discuss this with your doctor to ensure continued safety.
No data are available regarding the effects of methenamine hippurate on human fertility. If you are planning to become pregnant or have concerns about fertility, discuss this with your healthcare provider.
Driving and Operating Machinery
Hiprex has no known effect on the ability to drive or use machines. The medication does not cause drowsiness, dizziness, or impaired cognitive function at recommended doses.
Sodium Content
Hiprex 1 g oral powder (sachet formulation) contains less than 1 mmol (23 mg) of sodium per dose, which means it is essentially sodium-free. This is relevant for patients on sodium-restricted diets due to conditions such as heart failure, hypertension, or kidney disease. The tablet formulation also contains negligible amounts of sodium.
How Does Hiprex Interact with Other Drugs?
Drug interactions with Hiprex are primarily related to the pH-dependent mechanism of action of methenamine. Since the conversion of methenamine to bactericidal formaldehyde requires acidic urine (pH below 5.5), any medication or substance that raises urinary pH will reduce the effectiveness of Hiprex. Additionally, certain drug combinations pose a direct safety risk. It is essential to inform your doctor or pharmacist about all medications, supplements, and herbal products you are currently taking.
Major Interactions
| Interacting Drug | Effect | Clinical Significance |
|---|---|---|
| Sulfonamides (e.g., sulfamethoxazole, sulfasalazine) | Risk of crystalluria (insoluble crystal formation in urine) due to interaction between formaldehyde and sulfonamides | Absolute contraindication – never combine |
| Potassium citrate and sodium citrate mixtures | Alkalinizes the urine, preventing hydrolysis of methenamine to formaldehyde | Eliminates antibacterial effect – do not use concurrently |
| Sodium bicarbonate (baking soda) | Alkalinizes urine, preventing formaldehyde release | Avoid concurrent use; includes bicarbonate-containing products |
| Acetazolamide (carbonic anhydrase inhibitor) | Increases urinary pH through carbonic anhydrase inhibition | Significantly reduces Hiprex efficacy – avoid combination |
Minor Interactions
| Interacting Drug | Effect | Clinical Significance |
|---|---|---|
| Antacids (aluminium/magnesium hydroxide, calcium carbonate) | May modestly increase urinary pH, reducing formaldehyde release | Separate doses by at least 2 hours; monitor efficacy |
| Proton pump inhibitors (omeprazole, pantoprazole) | Minimal effect on urinary pH (act primarily on gastric acid) | Generally safe to combine; no significant interaction |
| Vitamin C (ascorbic acid) | May acidify urine, theoretically enhancing formaldehyde release | Limited clinical evidence; discuss with doctor before routine co-administration |
| Urinary steroid/catecholamine tests | Methenamine may cause falsely elevated results in certain urine assays | Inform laboratory staff about Hiprex use before testing |
In general, Hiprex has a favorable drug interaction profile compared to many antibiotics. Because methenamine is not metabolized by cytochrome P450 enzymes, it does not interact with the wide range of medications that affect CYP-mediated drug metabolism. The primary concern is maintaining acidic urinary pH and avoiding the sulfonamide combination. Patients are advised to maintain a balanced diet and avoid excessive consumption of strongly alkaline foods or drinks that could raise urinary pH.
What Is the Correct Dosage of Hiprex?
Always take Hiprex exactly as your doctor or pharmacist has instructed. Do not change the dose without consulting your healthcare provider. The tablet should be swallowed whole with a full glass of water. If you are using the oral powder formulation (sachet), dissolve the contents of one sachet in a glass of water and drink the solution. Regular timing of doses helps maintain consistent urinary antiseptic levels.
Adults
Standard Adult Dosage
Dose: 1 g (one tablet or one sachet) taken orally
Frequency: Two to three times daily
Duration: As directed by your doctor; Hiprex is typically used for long-term prophylaxis. Duration may range from several weeks to many months depending on the clinical indication.
For UTI prophylaxis, treatment is usually continued for as long as the patient remains at risk of recurrent infections. Your doctor will periodically review whether continued prophylaxis is necessary.
Children
Children Over 12 Years
Dose: 1 g (one tablet or one sachet) taken orally
Frequency: Twice daily
Duration: As directed by the prescribing physician
Children Aged 6–12 Years
Formulation: Hiprex 1 g tablets (dose adjusted by physician)
Dose: As determined individually by the prescribing doctor based on the child’s age, weight, and clinical need
Hiprex is not recommended for children under 6 years of age.
Elderly Patients
No specific dose adjustment is required for elderly patients with normal kidney and liver function. However, since renal function tends to decline with age, your doctor may assess kidney function before initiating Hiprex therapy and at regular intervals during treatment. Elderly patients should also ensure adequate hydration.
Missed Dose
If you forget to take a dose, take it as soon as you remember. However, if it is almost time for your next scheduled dose, skip the missed dose and continue with your regular dosing schedule. Do not take a double dose to make up for a forgotten dose. Maintaining regular dosing is important for consistent prophylactic protection.
Overdose
If you take more Hiprex than prescribed, the most common symptoms include nausea, vomiting, and potentially blood in the urine (hematuria). Dizziness and tinnitus (ringing in the ears) may also occur. In the event of an overdose, drink plenty of water to help dilute the formaldehyde concentration in the urine and flush the urinary tract. Seek immediate medical advice by contacting your doctor, going to the nearest hospital emergency department, or calling your local poison control center.
Hiprex works best when the urine is acidic (pH below 5.5). Avoid taking antacids, urinary alkalinizers, or large amounts of alkaline foods (such as citrus fruits in excess, milk, or bicarbonate of soda) around the time of your Hiprex dose. Maintain moderate, regular fluid intake – both too little and too much water can affect the drug’s effectiveness.
What Are the Side Effects of Hiprex?
Like all medicines, Hiprex can cause side effects, although not everybody gets them. The side effect profile of methenamine hippurate is generally mild compared to many antibiotics. Most adverse effects are dose-related and resolve when the medication is discontinued or the dose is reduced. The formaldehyde released in the bladder is the primary cause of local side effects such as bladder irritation, while systemic side effects are relatively uncommon at recommended doses.
The following side effects have been reported with Hiprex. They are organized by frequency according to the standard classification system:
Common
May affect up to 1 in 10 people
- Nausea
- Vomiting
- Skin rash (cutaneous hypersensitivity reactions)
- Bladder irritation (dysuria, urinary frequency, suprapubic discomfort)
Rare
May affect up to 1 in 1,000 people
- Hematuria (blood in the urine)
Understanding the Side Effects
The gastrointestinal side effects (nausea and vomiting) are typically related to the oral administration of the drug and can often be managed by taking Hiprex with food. If nausea persists, your doctor may advise dividing the daily dose differently or temporarily reducing the dose.
Bladder irritation occurs because formaldehyde, while bactericidal, can also irritate the bladder mucosa at higher concentrations. This is more likely to occur in patients who are dehydrated or who take higher-than-recommended doses. Maintaining adequate fluid intake helps keep formaldehyde concentrations at effective but non-irritating levels. Symptoms of bladder irritation include a burning sensation during urination, increased urinary frequency, and mild suprapubic discomfort.
Skin rash is an uncommon hypersensitivity reaction. If you develop a rash while taking Hiprex, discontinue the medication and consult your doctor. In most cases, the rash resolves promptly after stopping the drug.
Hematuria (blood in the urine) is a rare side effect that may indicate excessive formaldehyde concentration causing irritation of the bladder or urinary tract lining. If you notice blood in your urine, stop taking Hiprex and contact your healthcare provider immediately.
It is worth noting that the overall safety profile of methenamine hippurate has been established over many decades of clinical use. The ALTAR trial (2022), which followed patients taking methenamine hippurate for 12 months, reported a low overall incidence of adverse events, and the side effect profile was comparable to or better than that of daily low-dose antibiotic prophylaxis.
If you experience any side effects, including those not listed here, tell your doctor or pharmacist. You can also report suspected side effects to your national pharmacovigilance authority (e.g., the EMA in Europe, the FDA MedWatch program in the United States, or the MHRA Yellow Card Scheme in the United Kingdom) to help continuously monitor the benefit-risk balance of this medicine.
How Should Hiprex Be Stored?
Proper storage of medications is important to ensure they remain effective and safe throughout their shelf life. Follow these guidelines for storing Hiprex:
- Original packaging: Keep the tablets in their original blister pack or container to protect them from moisture. Methenamine hippurate is moisture-sensitive, and exposure to humidity can reduce the stability of the product.
- Temperature: Store at room temperature. No special temperature requirements beyond normal household conditions.
- Keep out of reach of children: Store this medicine in a safe place where children cannot see or reach it.
- Expiry date: Do not use Hiprex after the expiry date stated on the packaging. The expiry date refers to the last day of that month.
- Disposal: Do not dispose of medicines via household waste or wastewater. Return unused or expired medication to your pharmacy for proper disposal. These measures help protect the environment.
What Does Hiprex Contain?
Active Substance
The active substance is methenamine hippurate, present at a dose of 1 gram per tablet or per sachet. Methenamine hippurate is a salt formed from methenamine (hexamethylenetetramine) and hippuric acid. The molecular formula is C15H21N5O3, and it has a molecular weight of approximately 319.36 g/mol. Methenamine itself is a white crystalline powder that is highly soluble in water.
Inactive Ingredients (Excipients)
The oral powder sachet formulation contains the following inactive ingredients:
- Saccharin sodium (artificial sweetener)
- Lemon flavoring
The tablet formulation may contain additional excipients such as magnesium stearate and povidone, which serve as lubricants and binders during the manufacturing process. The exact excipient composition may vary slightly between manufacturers. Consult the patient information leaflet supplied with your specific product for the complete list of ingredients.
Manufacturer
Hiprex is marketed by Viatris (formerly Mylan) in many markets. Manufacturing sites include Fine Foods & Pharmaceuticals N.T.M. S.p.A. in Brembate, Italy, and Mylan Hungary Kft. in Komarom, Hungary. The specific manufacturer for your product can be found on the packaging. Other brands of methenamine hippurate, such as Methenamine hippurate Orion, may be manufactured by different companies but contain the same active substance.
Frequently Asked Questions About Hiprex
Hiprex (methenamine hippurate) is a urinary antiseptic used to prevent recurrent urinary tract infections (UTIs) in people who are prone to repeated infections. It is also used during urological procedures and investigations to reduce infection risk, and in patients who require long-term urinary catheterization. Hiprex works by releasing formaldehyde in acidic urine, which kills bacteria in the urinary tract without promoting antibiotic resistance.
Hiprex contains methenamine hippurate, which is broken down (hydrolyzed) in acidic urine to produce formaldehyde. Formaldehyde is a broad-spectrum bactericidal agent that kills most common urinary tract pathogens by denaturing their proteins and nucleic acids. The hippuric acid component also helps acidify the urine, creating more favorable conditions for formaldehyde release. This mechanism provides a non-antibiotic approach to UTI prevention, which is important for reducing antimicrobial resistance.
No, Hiprex is not a conventional antibiotic. It is classified as a urinary antiseptic. Unlike antibiotics, which target specific bacterial mechanisms, Hiprex works by releasing formaldehyde in acidic urine, which non-specifically denatures bacterial proteins and nucleic acids. Because of this non-specific mechanism, bacteria do not develop resistance to methenamine the way they can to traditional antibiotics. This makes Hiprex a valuable alternative for long-term UTI prophylaxis, especially in the era of increasing antimicrobial resistance.
No adverse effects on the fetus have been demonstrated with methenamine hippurate, and it has a long history of use during pregnancy. However, you should always consult your doctor before taking any medication during pregnancy. The drug also passes into breast milk but is unlikely to affect the breastfed infant at recommended doses. For more than occasional use during breastfeeding, discuss the risks and benefits with your doctor.
Cranberry juice and vitamin C (ascorbic acid) may theoretically help by acidifying the urine, which enhances formaldehyde release from methenamine. However, clinical evidence for these specific combinations is limited, and the urinary acidifying effect of cranberry juice is modest at best. It is more important to avoid substances that alkalinize the urine, such as antacids, sodium bicarbonate, or potassium citrate mixtures, as these will reduce the effectiveness of Hiprex. Always discuss any supplements or dietary changes with your doctor.
Hiprex can be taken for extended periods as determined by your doctor. For UTI prophylaxis, treatment may continue for several months to a year or longer, depending on your risk of recurrent infections. The ALTAR trial demonstrated that methenamine hippurate was effective and well tolerated over a 12-month treatment period. Your doctor will periodically review whether continued prophylaxis is necessary. Unlike long-term antibiotic use, long-term Hiprex use does not promote antimicrobial resistance, which is one of its major advantages.
References
- Harding C, Mossop H, Homer T, et al. Alternative to prophylactic antibiotics for the treatment of recurrent urinary tract infections in women: multicentre, open label, randomised, non-inferiority trial (ALTAR). BMJ. 2022;376:e068229. doi:10.1136/bmj-2021-068229.
- Lee BSB, Bhuta T, Simpson JM, Craig JC. Methenamine hippurate for preventing urinary tract infections. Cochrane Database Syst Rev. 2012;(10):CD003265. doi:10.1002/14651858.CD003265.pub3.
- European Association of Urology (EAU). EAU Guidelines on Urological Infections. 2024 Edition. Available from: EAU Guidelines.
- National Institute for Health and Care Excellence (NICE). Urinary tract infection (recurrent): antimicrobial prescribing. NICE Guideline NG112. Updated 2024. Available from: NICE NG112.
- World Health Organization (WHO). WHO Model List of Essential Medicines – 23rd List. Geneva: WHO; 2023.
- European Medicines Agency (EMA). Methenamine hippurate – Summary of Product Characteristics. Available from: EMA.
- Foxman B. Urinary tract infection syndromes: occurrence, recurrence, bacteriology, risk factors, and disease burden. Infect Dis Clin North Am. 2014;28(1):1–13. doi:10.1016/j.idc.2013.09.003.
- Botto H, Neuzillet Y. Methenamine hippurate (Hiprex): an old drug revisited for an emerging problem. Eur Urol Focus. 2023;9(1):10–12. doi:10.1016/j.euf.2022.10.011.
- Anger J, Lee U, Ackerman AL, et al. Recurrent uncomplicated urinary tract infections in women: AUA/CUA/SUFU guideline. J Urol. 2019;202(2):282–289. doi:10.1097/JU.0000000000000296.
- Stothers L. A randomized trial to evaluate effectiveness and cost effectiveness of naturopathic cranberry products as prophylaxis against urinary tract infection in women. Can J Urol. 2002;9(3):1558–1562.
Editorial Team
This article was written and reviewed by the iMedic Medical Editorial Team, comprising licensed specialist physicians with expertise in urology, infectious disease, and clinical pharmacology.
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