Methenamine

Methenamine (Hexamethylenetetramine): A Comprehensive Reference

1. Identity: Chemical Names, Structure, and Common Forms

Hexamethylenetetramine, also known as methenamine, urotropin, or hexamine, is a white, odorless, crystalline solid that is a versatile heterocyclic organic compound with the molecular formula C₆H₁₂N₄ and the IUPAC name 1,3,5,7-tetraazatricyclo[3.3.1.1³,⁷]decane. It features a symmetric, cage-like polycyclic structure resembling adamantane, in which the carbon atoms at positions 1, 3, 5, and 7 are substituted by nitrogen atoms, making it the first known polycyclic amine.

Methenamine is also known as hexamine, hexamethylenetetramine, urotropine, and aminoform. The compound is highly soluble in water (approximately 850 g/L at 20°C) and sublimes at approximately 280°C without melting, with low volatility and a density of 1.33 g/cm³.

In pharmaceutical contexts, methenamine is not administered as the free base alone. It is provided pharmaceutically as the hippuric acid salt (methenamine hippurate) and the mandelic acid salt (methenamine mandelate). The free base and other salts, including anhydromethylencitrate and sulfosalicylate, have also been marketed in some countries.

Methenamine hippurate is the salt formed from methenamine (C₆H₁₂N₄) and hippuric acid (C₉H₉NO₃). Chemically, methenamine hippurate is the hippuric acid salt of methenamine (hexamethylenetetramine).

Common Brand Names and Regulatory Status

Methenamine is sold under the brand names Hiprex, Urex, and Urotropin, among others. Methenamine is approved and available in the United States. Only methenamine hippurate, the twice-daily formulation, is available as a prescription drug in the United States.

The drug name methenamine, a contraction of the chemical or scientific name hexamethylenetetramine, was formally introduced and designated by the United States Pharmacopeia (USP) by 1925 and replaced the prior name of the drug that was being used of hexamethylenamine. The alternative drug name hexamine was introduced in the British Pharmacopoeia (BP) by 1914 to be used instead of the commercial name Urotropin.

Pharmaceutical Forms and Dosage Formulations

Methenamine mandelate is provided as an enteric coated tablet and is taken four times daily, whereas methenamine hippurate is available only in non-coated tablet form and is taken twice daily. Methenamine hippurate is more popular and commonly used owing to its more convenient dosing schedule. Methenamine is taken three times daily in the case of formulations in which low-dose methenamine free base is combined with sodium salicylate.

Methenamine, in a topical cream or gel stick formulation sold under brand names like Antihydral and Dehydral, is used in the treatment of hyperhidrosis (excessive sweating) and has been reported to be clinically effective for this indication. The topical form of methenamine for hyperhidrosis has been marketed only in certain countries, including Austria, Canada, Germany, Luxembourg, and Switzerland.

Methenamine is available both alone and in combination with the nonsteroidal anti-inflammatory drug (NSAID) sodium salicylate. Other combinations with other drugs, such as phenazopyridine, are also available.

2. Natural Sources and Chemical Origin

Methenamine is not derived from a botanical or naturally occurring biological source; it is a fully synthetic organic compound. Discovered in 1859 by Russian chemist Aleksandr Butlerov during studies of formaldehyde-ammonia reactions, hexamethylenetetramine was the inaugural example of a polycyclic amine and has since become a key industrial chemical produced via the exothermic condensation of formaldehyde and excess ammonia in aqueous solution.

However, it is classified in pharmacological literature as a prodrug that exploits a naturally acidic physiological environment — the urinary bladder — to generate its active principle. It is prepared industrially by combining formaldehyde and ammonia. Hippuric acid, the acid component of the most clinically used salt form, is itself found naturally in human urine and in many foods, particularly those containing benzoic acid or its precursors.

3. Historical Use and Discovery

Methenamine was discovered in 1859 and was first used as a urinary antiseptic medicine in 1894. It was introduced for medical use as a urinary antiseptic under the name Urotropin in 1895. The drug was described as rapidly sterilizing and thereby restoring putrid and pus-filled urine to a normal appearance and constitution.

A combination of methenamine with salicylic acid was also developed and introduced the same year. Methenamine was only used as a urinary antiseptic in cases of acidic urine, whereas boric acid was used to treat UTIs with alkaline urine.

Methenamine's use as a urinary antiseptic predates the antibiotic era by nearly half a century. Methenamine is a urinary antiseptic first introduced in 1895. Over the first half of the twentieth century it was a primary pharmaceutical option for urinary infection management. With the advent of sulfonamides in the 1930s and penicillin in the 1940s, methenamine gradually receded from prominence as antibiotics displaced it. In the era of multidrug resistance, it is critical to utilize antibiotics in an appropriate manner and to identify new treatments or revisit the use of 'forgotten' drugs. Because urinary tract infections (UTIs) are common, particularly in an increasing elderly population, the 'forgotten' drug, methenamine, may become important as a preventive therapy for recurrent UTIs.

Methenamine was approved for use in the United States in 1976 but is not widely used.

Beyond urinary medicine, methenamine has a documented industrial and culinary history. As a food additive, hexamine is used as a preservative in cheeses. It has E number E239. In some countries it is banned for this use. Together with 1,3,5-trioxane, hexamethylenetetramine is a component of hexamine fuel tablets used by campers, hobbyists, the military and relief organizations for heating camping food or military rations.

4. Key Constituents and Active Compounds

Methenamine itself is the primary and sole pharmacologically relevant molecule, and it functions as a prodrug. The biologically active agent it generates is formaldehyde. In the hippurate salt formulation, hippuric acid also contributes pharmacologically.

Methenamine (Hexamethylenetetramine)

Methenamine itself is a prodrug that remains inactive until it encounters an acidic environment, which is typically found in the urine. The molecule's cage-like structure containing four nitrogen atoms enables it to travel through the bloodstream and gastrointestinal tract intact without significant premature hydrolysis, delivering its active metabolite selectively to the urinary tract.

About 10–30% of an oral dose is hydrolyzed by gastric acidity to formaldehyde and ammonia. Enteric coating of methenamine mandelate tablets reduces hydrolysis in the GI tract and rate of absorption.

Formaldehyde (the Active Antimicrobial Metabolite)

In an acidic environment, methenamine is converted to ammonia and formaldehyde, which inhibits prokaryotic cell division and denatures bacterial proteins and nucleic acids. Formaldehyde is a potent antiseptic, and its antimicrobial activity is broad-spectrum, meaning it is effective against a wide range of bacteria. It works by denaturing bacterial proteins and DNA, ultimately leading to cell death.

Hippuric Acid (in Methenamine Hippurate)

The hippuric acid component maintains urinary pH acidity, creating an unfavourable environment for bacterial growth while enhancing formaldehyde formation. Hippuric acid, the other component, has some antibacterial activity and also acts to keep the urine acid.

5. Mechanisms of Action

Prodrug Hydrolysis and Formaldehyde Release

When methenamine hippurate is ingested, it is absorbed through the gastrointestinal tract and then excreted through the kidneys into the urine. In the acidic environment of the urine (usually with a pH of 5.5 or lower), methenamine is hydrolyzed, releasing formaldehyde. This hydrolysis reaction is facilitated by the acidic pH, which acts as a catalyst.

UREX (methenamine hippurate) exerts its activity because the methenamine component is hydrolyzed to formaldehyde in acid urine. Hippuric acid, the other component, acts to keep the urine acid.

Antimicrobial Spectrum

Formaldehyde exhibits broad-spectrum antimicrobial activity whereby it denatures proteins and nucleic acids of bacteria such as many uropathogens, including Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis; however, Enterococcus is generally resistant. The drug is generally active against E. coli, enterococci and staphylococci. Enterobacter aerogenes is generally resistant. The urine must be kept sufficiently acid for urea-splitting organisms such as Proteus and Pseudomonas to be inhibited.

Resistance Profile

Despite over a century of use, there is no evidence of bacterial resistance to methenamine's bacteriostatic activity. Formaldehyde is directly bactericidal. Almost all bacteria are sensitive to formaldehyde and resistance to it does not develop. This distinguishes methenamine fundamentally from conventional antibiotics, whose extended use drives selective resistance pressure.

pH Dependency and Limitations

Methenamine is converted into formaldehyde only in acidic environments like the urinary bladder and hence is not expected to be effective in the eradicative treatment of pyelonephritis (kidney infection) or chronic bacterial prostatitis. As a result, it is not recommended for such indications.

It is not considered effective in treating acute urinary infections. Its role is therefore confined to prophylaxis (prevention) of recurrent infection in the lower urinary tract.

Effect on the Urinary Microbiome

As its mechanism of action is primarily bacteriostatic, methenamine may preserve protective microbial diversity. Recently, Acevedo-Alvarez and colleagues evaluated the longitudinal effect of MH on the urobiome of 6 postmenopausal women with recurrent UTIs. From each participant, they obtained voided and catheterized urine samples as well as periurethral swabs daily for 1 week before and 3 months after starting MH. MH treatment increased the richness of the urobiome in catheterized urine specimens with little change in evenness or overall diversity.

6. Body Systems and Health Areas Associated with Methenamine

Urinary Tract (Primary Application)

The overwhelming body of research on methenamine concerns the lower urinary tract — specifically, the prevention of recurrent urinary tract infections (rUTIs). Methenamine is a urinary tract antiseptic and antibacterial drug used for the prophylaxis and treatment of frequently recurring urinary tract infections requiring long-term therapy.

Dermatological (Hyperhidrosis)

Methenamine, in a topical cream or gel stick formulation, is used in the treatment of hyperhidrosis (excessive sweating) and has been reported to be clinically effective for this indication. The skin is slightly acidic and formaldehyde can be released from methenamine in this environment.

Gastrointestinal (Speculative/Exploratory)

Methenamine might be useful in the treatment of Helicobacter pylori infections as it is activated in the acidic environment of the stomach. This application remains speculative, and no robust clinical trials have been published to support this use.

7. Scientific Evidence by Area of Use

7.1 Prevention of Recurrent Urinary Tract Infections

This is the principal evidence-based application of methenamine. The evidence base spans early randomized controlled trials from the 1970s and 1980s, a Cochrane systematic review, multiple more recent systematic reviews and meta-analyses, and a landmark pragmatic non-inferiority trial.

Early Randomized Controlled Trials

One randomized controlled trial from 1982 by Kasanen et al. allocated patients with recurrent UTIs to one of 4 groups: placebo, nitrofurantoin 75 mg, methenamine hippurate 1 g, or trimethoprim 100 mg. At one year, 63.2% of those in the placebo group had a recurrence, compared to 34.2% in the methenamine hippurate group, 25% in the nitrofurantoin group, and 10.4% in the trimethoprim group.

A crossover study done by Cronberg et al. found that the recurrence rate was reduced to 0.8 UTIs/patient/year using long-term methenamine hippurate prophylaxis.

Cochrane Systematic Review (2012)

Methenamine hippurate may be effective for preventing UTI in patients without renal tract abnormalities, particularly when used for short-term prophylaxis. It does not appear to work in patients with neuropathic bladder or in patients who have renal tract abnormalities. The rate of adverse events was low, but poorly described. There is a need for further large well-conducted RCTs to clarify this question, particularly for longer-term use for people without neuropathic bladder.

Systematic Review and Meta-Analysis (British Journal of General Practice, 2021)

Six studies involving 557 participants were included (447 were analysed). Of the six studies, five were published and one was an unpublished trial record with results, three compared methenamine hippurate against placebo or control, and three compared methenamine hippurate with antibiotics.

For the number of patients who remained asymptomatic, methenamine hippurate showed a non-statistically significant trend of benefit versus antibiotics over 12 months (risk ratio [RR] 0.65, 95% confidence interval [CI] = 0.40 to 1.07, I² 49%), versus control over 6 or 12 months (RR 0.56, 95% CI = 0.13 to 2.35, I² 93%), and a non-statistically significant trend versus any antibiotic for abacteruria (RR 0.80, 95% CI = 0.62 to 1.03, I² 23%).

Overall, the insufficiency of evidence precluded a firm recommendation on the use of methenamine hippurate prophylactically; however, there was enough evidence to warrant further research to investigate its benefits.

The ALTAR Trial (Non-Inferiority RCT, NIHR, 2022)

The ALTAR trial (Alternative to Prophylactic Antibiotics for the Treatment of Recurrent Urinary Tract Infections in Women) represents the most significant clinical evidence to date. The ALTAR trial is a multicentre, pragmatic, open-label randomised non-inferiority trial evaluating the clinical effectiveness and cost-effectiveness of the urinary antiseptic methenamine hippurate, a non-antibiotic treatment for the prevention of rUTI, and comparing it with the current standard treatment of low-dose antibiotic prophylaxis.

Design: Multicentre, pragmatic, open-label, randomised, non-inferiority trial of 12 months' treatment with the allocated intervention, including an early, embedded qualitative study and a 6-month post-treatment observation phase. The predefined non-inferiority margin was one urinary tract infection per person-year. Setting: Eight UK NHS secondary care sites. Participants: A total of 240 adult women with recurrent urinary tract infection requiring preventative treatment participated in the trial.

The inclusion criteria for the trial were to have experienced recurrent UTI, defined as at least three episodes of symptomatic UTI in the last 12 months, two episodes in the last 6 months, or one episode requiring hospitalisation.

This trial showed that methenamine hippurate is not inferior to the current standard care of daily low-dose antibiotics in preventing recurrent urinary tract infections in women during 12 months of prophylactic treatment. The results suggest that antimicrobial resistance is proportionally higher in women taking prophylactic antibiotics.

The robust evaluation of a non-antibiotic preventative treatment for women with recurrent uncomplicated UTIs provided by this trial represents a high level of supportive evidence for the routine use of methenamine hippurate for this indication.

Updated Systematic Review and Meta-Analysis (BMC Urology, 2025)

This updated systematic review and meta-analysis aimed to evaluate the effectiveness of methenamine hippurate in preventing UTIs, incorporating the latest research findings and employing trial sequential analysis to assess the robustness of the evidence. A systematic review was conducted across MEDLINE, Embase, Scopus, Cochrane, and Google Scholar up to March 2024 for randomized controlled trials comparing methenamine hippurate with placebo or antibiotic in adult women with a history of recurrent, confirmed UTIs. Key outcomes included symptomatic UTIs as primary outcome and positive urine culture, asymptomatic bacteriuria and adverse effects as secondary outcomes.

Observational Study in Patients with Structural Abnormalities (South Wales, 2025)

A retrospective observational study was conducted on the first 150 patients prescribed MH in a University Hospital at a Local Health Board in South Wales from April 2020 to July 2022. After 12 weeks of treatment, 100 (66.7%) of patients showed improvement, with about half of these (n = 46) experiencing complete resolution of UTIs and a further third (n = 35) reporting reduced frequency or severity at six months follow-up.

Effect on Antibiotic Resistance

A distinctive finding from comparative studies is the relative effect of each strategy on antimicrobial resistance. Methenamine can reduce the risk of UTIs by 44 to 86% and has been found to be non-inferior to low-dose prophylactic antibiotics. It is not an antibiotic, and unlike antibiotics, has no risk of bacterial resistance. The ALTAR trial specifically quantified this: the results suggest that antimicrobial resistance is proportionally higher in women taking prophylactic antibiotics.

Comparison with Nitrofurantoin

Using nitrofurantoin as prophylaxis for the prevention of recurrent UTI significantly reduced the incidence of UTI in adults and children compared with methenamine hippurate (2 RCTs, n=196: 35.8% versus 51.2%; RR 0.60, 95% CI 0.43 to 0.85; NNT 7 [95% CI 4 to 102]; low quality evidence). This finding of nitrofurantoin superiority over methenamine hippurate is based on low-quality evidence and should be interpreted cautiously.

Systematic Review in Women (2023)

Relevant databases were searched for RCTs using Cochrane methodology and reporting items for systematic reviews and meta-analyses (PRISMA) checklist, comparing the efficacy of methenamine hippurate to either an antibiotic or a placebo for the prophylaxis of rUTI in women. Six trials involving 322 patients taking methenamine and 419 patients receiving antibiotics in total were evaluated. The duration of the trials ranged from 12 to 24 months.

Guideline Endorsements (2024–2025)

On September 4, 2025, the American Urological Association (AUA), in collaboration with the Canadian Urological Association and the Society of Urodynamics, Female Pelvic Medicine & Urogenital Reconstruction (SUFU), released an amendment. The guideline amendments are based on a review of the literature that included 87 studies published between June 2021 and November 2024. The updated guideline includes an expansion of nonantibiotic options for UTI prophylaxis. Clinicians are now advised to offer cranberry, methenamine hippurate, and increased water intake for patients with a water intake below 1.5 L (50 oz) per day as prophylaxis options for women with recurrent UTIs.

Overall Strength of Evidence for UTI Prevention

The evidence base for methenamine hippurate as a UTI prophylactic is now substantial but carries important caveats. The ALTAR trial provides the most methodologically rigorous evidence to date for non-inferiority to antibiotic prophylaxis in women with uncomplicated recurrent UTIs. Earlier trials were generally small, older, and of variable methodological quality. Methenamine does not appear to work in patients with neuropathic bladder or in patients who have renal tract abnormalities. Methenamine mandelate and hippurate are effective in the prevention of recurrent urinary tract infections except in patients with Foley catheters or who require intermittent catheterization.

7.2 Topical Use for Hyperhidrosis

Sixty patients with palmo-plantar hyperhidrosis were studied to compare the efficacy and safety of topical methenamine in the treatment of palmo-plantar hyperhidrosis with established therapies like glutaraldehyde and tap water iontophoresis. Patients were randomly allocated to 3 treatment groups: topical methenamine (10%) solution, topical glutaraldehyde (5% for palms and 10% for soles), and tap water iontophoresis. The total duration of therapy was 4 weeks for all 3 groups. Patients were followed up weekly for 4 weeks and 2 weeks after completion of therapy.

The evidence base for topical methenamine in hyperhidrosis is limited to small controlled trials and observational data; it is not considered a standard of care in most countries and regulatory approval for this indication is geographically restricted.

8. Pharmacokinetics

UREX is readily absorbed from the GI tract. Methenamine distributes widely into body fluids, but very little is hydrolyzed prior to excretion in the kidney and thus has minimal systemic toxic effects. Methenamine is placentally transferred to the fetus during pregnancy.

90% of the methenamine moiety is excreted in the urine within 24 hours after administration of a single 1-gram dose. Similarly, the hippurate moiety is rapidly absorbed and excreted, and it reaches the urine by both tubular secretion and glomerular filtration.

MH has minimal systemic metabolism and a half-life of approximately four hours. Methenamine hippurate has a half-life of 4.5 hours, and reaches the antimicrobial concentration (MIC of formaldehyde) of 13 micrograms/mL 30 minutes to 1 hour after oral intake. A single 2-gram dose will yield 18–60 micrograms/mL (with typical daily doses as one dose every 12 hours).

Following oral administration of a usual single dose to healthy fasting adults, concentrations of methenamine and formaldehyde in plasma are generally very low and antibacterial activity in plasma is negligible. This is pharmacologically significant: systemic exposure to formaldehyde is minimal, and the active conversion occurs selectively in acidic urine.

9. Dosage Forms and Doses Reported in Studies

Methenamine is available generically as either a mandelate or hippuric salt in tablets of 1000 mg. The typical dose in adults is 1 gram twice daily (methenamine hippurate) to four times daily (methenamine mandelate).

The FDA-approved label for methenamine hippurate (Hiprex/Urex) specifies the following dosing:

• For adults: 1 g orally twice daily (BID), indicated for frequently recurring UTIs if long-term treatment is needed; urinary acidification may be necessary.
• For children aged 6–12 years: 0.5–1 g orally twice daily; for children over 12 years: 1 g orally twice daily.

Each white, scored UREX tablet contains methenamine hippurate 1 g.

In clinical trials, the dose used is consistently 1 g twice daily for methenamine hippurate. The randomized controlled trial by Kasanen et al. (1982) used methenamine hippurate 1 g as its study dose. The ALTAR trial similarly used 1 g twice daily of methenamine hippurate (Hiprex) as the intervention arm.

In studies of hyperhidrosis, topical solutions at a concentration of 10% have been used. Topical methenamine at a 10% solution was used in the palmo-plantar hyperhidrosis study.

Regarding large doses associated with harm: large doses (8 g daily for 3–4 weeks) have caused bladder irritation, painful and frequent micturition, albuminuria, and gross hematuria. This substantially exceeds the recommended therapeutic dose.

10. Safety Considerations and Drug Interactions

Adverse Effects

Adverse effects of UREX (methenamine hippurate) have been reported in fewer than 3.5% of patients treated. These reactions have included the following, in decreasing order of frequency: nausea, vomiting, and rarely pruritus, rash, dysuria.

GI disturbances (nausea, vomiting, diarrhea, abdominal cramps, anorexia), pruritus, rash, and dysuria are the main reported adverse effects.

Transient elevations in serum AST and ALT concentrations have occurred in patients receiving methenamine hippurate. Periodic liver function tests should be performed in patients receiving methenamine hippurate, especially in those with hepatic impairment.

The rate of adverse events was low, but poorly described across the earlier literature. In the ALTAR trial, only 2 serious adverse reactions were identified — both occurring in the antibiotic group rather than the methenamine hippurate group.

Contraindications

HIPREX (methenamine hippurate tablets USP) is contraindicated in patients with renal insufficiency, severe hepatic insufficiency, or severe dehydration.

Patients with pre-existing hepatic insufficiency may suffer adverse effects from the small amounts of ammonia and formaldehyde that are produced.

Drug Interactions

Sulfonamide antibiotics: Methenamine preparations should not be given to patients taking sulfonamides because some sulfonamides may form an insoluble precipitate with formaldehyde in the urine.

Alkalizing agents: Alkalizing substances (such as antacids) reduce the effect of methenamine hippurate and should be avoided. Because the entire mechanism of action depends on an acidic urinary pH, any agent that raises urine pH substantially will negate methenamine's antimicrobial activity.

Urea-splitting organisms: Care should be taken to maintain an acid pH of the urine, especially when treating infections due to urea-splitting organisms such as Proteus and strains of Pseudomonas.

Laboratory test interference: Methenamine causes spuriously elevated urinary 17-hydroxycorticosteroid and catecholamine levels. This medication can interfere with tests of urine estriol in pregnancy resulting in unmeasurably low values when acid hydrolysis is used in the laboratory procedure; testing which utilizes enzymatic hydrolysis is unaffected.

Pregnancy and Lactation

Methenamine is placentally transferred to the fetus during pregnancy. In early pregnancy the safe use of HIPREX is not established. In the last trimester, safety is suggested, but not definitely proved. No adverse effects on the fetus were seen in studies in pregnant rats and rabbits.

Methenamine is distributed into breast milk in low concentrations; the manufacturers recommend caution.

Carcinogenicity Concerns

Because of the known link between formaldehyde and cancer (specifically nasopharyngeal cancer or leukemia), there have been concerns about methenamine's carcinogenic potential. Although no studies have looked directly at the long-term effects, no case reports document cancer arising as a result of methenamine use. Also, animal models have shown no evidence of carcinogenicity or increase in neoplasm rates when given methenamine orally. The rationale for the low concern is that systemic formaldehyde exposure from therapeutic oral doses is minimal, given the selective hydrolysis in the acidic urinary environment rather than in systemic circulation.

Renal Impairment

Methenamine 90% is excreted within 24 hours; mandelic or hippuric acid may accumulate with severe renal impairment. For this reason, the drug is contraindicated in patients with renal insufficiency.

Antimicrobial Stewardship Context

While antibiotics remain the mainstay of UTI treatment, with proven efficacy, growing concerns about antimicrobial resistance (AMR) highlight the need for alternative strategies. Methenamine hippurate occupies a unique position in this landscape: it causes few side effects, kills bacteria by denaturing bacterial proteins, RNA, and DNA, and does not develop resistance. A 2024 study found that methenamine was more cost-effective than low-dose prophylactic antibiotics for prevention of UTIs.

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