Methenamine hippurate is a urinary antiseptic used to prevent recurrent urinary tract infections (UTIs). Unlike antibiotics, it doesn’t kill bacteria directly. Instead, it breaks down into formaldehyde inside your bladder, creating an environment where bacteria can’t thrive. It’s sold under brand names like Hiprex and Urex and has been used in Scandinavian countries for nearly 50 years as a preventive treatment for people who get frequent UTIs.
The key distinction: methenamine hippurate is for prevention, not treatment. It won’t clear an active infection, but taken regularly, it can reduce how often infections come back.
How It Works in Your Body
After you swallow a tablet, methenamine hippurate is absorbed through your digestive tract and filtered out by your kidneys into your urine. Once there, the compound splits into two parts: methenamine and hippuric acid. If your urine is acidic enough, the methenamine converts into formaldehyde, a potent chemical that denatures bacterial enzymes, essentially disabling the machinery bacteria need to survive and multiply. The hippuric acid component helps by contributing to that acidic environment your urine needs for the reaction to happen.
This chemical conversion is highly dependent on urine pH. Research on the conversion kinetics found that unless urine stays below a pH of 6, only a small fraction of methenamine actually converts to formaldehyde. That’s a critical detail because it means the drug works well only when your urine is consistently acidic. If your urine tends to be more alkaline, the medication may not generate enough formaldehyde to be effective.
Who Should Take It
Methenamine hippurate is primarily prescribed for women (and occasionally men) who experience recurrent UTIs, typically defined as two or more infections in six months or three or more in a year. It’s designed for long-term use as a preventive measure. In clinical trials, participants typically take 1 gram twice daily for six months or longer.
It is not appropriate for everyone. The FDA label lists clear contraindications: kidney insufficiency, severe liver disease, and severe dehydration. The liver restriction exists because methenamine generates ammonia as a byproduct, which the liver normally clears. If your liver can’t handle that load, ammonia can build up to dangerous levels. You also cannot take methenamine hippurate alongside sulfonamide antibiotics, because sulfonamides can form an insoluble precipitate with formaldehyde in the urine.
How It Compares to Daily Antibiotics
A large randomized trial conducted in the UK’s National Health Service (the ALTAR trial) directly compared methenamine hippurate to daily low-dose antibiotics in women with recurrent UTIs. During the treatment period, women taking methenamine hippurate experienced about 1.38 UTI episodes per year, compared to 0.89 episodes per year in the antibiotic group. That’s a difference of roughly half an extra infection per year, which fell within the study’s predefined threshold for non-inferiority. The conclusion: methenamine hippurate is a reasonable alternative to daily antibiotics for prevention.
The more interesting finding involved antibiotic resistance. During treatment, 72% of women on daily antibiotics showed resistant bacteria on perineal swabs, compared to 56% in the methenamine hippurate group. Women taking antibiotics also had higher rates of resistance to specific drug classes, including trimethoprim and cephalosporins. This matters because if you do develop a breakthrough UTI while on preventive antibiotics, the infection may be harder to treat.
There was one caveat. After treatment ended, the UTI rate in the methenamine hippurate group rose to 1.72 episodes per year, while the antibiotic group stayed lower at 1.19. This suggests methenamine hippurate’s protective effect fades once you stop, without the same carryover benefit some antibiotics provide. Adverse events were similar between both groups.
Keeping Your Urine Acidic Enough
Because the drug only works in acidic urine, what you eat and drink while taking it matters more than with most medications. Alkalizing foods and medications can raise your urine pH and reduce the drug’s effectiveness. This is especially important if your infection involves urea-splitting organisms like Proteus or certain Pseudomonas strains, which naturally make urine more alkaline.
Some practitioners recommend vitamin C (ascorbic acid) or cranberry juice to help acidify urine while on methenamine. Both have been used for this purpose for decades, though the degree to which they lower urine pH varies from person to person. The general principle is straightforward: avoid antacids, limit high-alkaline foods, and consider acidifying supplements if your urine pH tends to run above 6. You can track your urine pH inexpensively with over-the-counter test strips.
Side Effects
Methenamine hippurate is generally well tolerated. The most commonly reported issues are gastrointestinal: nausea, upset stomach, and occasionally a skin rash. Because formaldehyde is generated in the bladder, some people experience bladder irritation, particularly at higher doses or with prolonged use. These symptoms tend to be mild and resolve when the dose is adjusted or the medication is stopped.
Serious side effects are rare but relate to the drug’s core chemistry. In people with compromised kidney function, the drug and its byproducts can accumulate rather than being properly excreted. In people with liver disease, the ammonia generated during the breakdown process can reach harmful levels. These are the reasons behind the contraindications, not theoretical risks but predictable consequences of how the drug is metabolized.
Why It’s Gaining Attention Now
Methenamine hippurate isn’t new, but growing concern about antibiotic resistance has renewed interest in it as an alternative to long-term preventive antibiotics. Several large trials, including the ALTAR trial and the ongoing ImpresU trial in older women, are building the evidence base for broader use outside Scandinavia, where it has long been a standard option. The appeal is simple: it reduces UTI frequency without contributing to the same resistance patterns that daily antibiotics create. For people who get frequent UTIs and want to avoid months or years of antibiotic use, it offers a meaningfully different approach to prevention.