Is an Old Antibiotic the Best New Thing to Prevent UTIs?

Dr. Sharon Orrange
Dr. Orrange is an Associate Professor of Clinical Medicine in the Division of Geriatric, Hospitalist and General Internal Medicine at the Keck School of Medicine of USC.
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Urinary tract infections (UTIs) are common infections, especially in women. Recurrent UTIs are defined as 3 episodes of a UTI in the previous year or 2 episodes in the previous 6 months. In the primary care setting, 53% of women over 55 report a recurrence of UTI within a year.

As people with recurrent UITs are repeatedly treated with Bactrim (sulfamethoxazole/trimethoprim), ciprofloxacin, or nitrofurantoin, we are seeing more and more UTIs with resistant organisms. Multidrug-resistant (MDR) bacteria are now fairly common culprits of recurrent UTI, creating an urgent need for better options than the standard antibiotics.

Enter methenamine. Methenamine should be considered when the more commonly used antibiotics fail to prevent recurrent urinary tract infections. It has benefits as it can help prevent UTIs in people who are susceptible, and it can help with treatment of recurrent chronic UTI.

What’s methenamine?

Methenamine (brand names Hiprex or Urex) has been used to treat bacterial infection in patients with acute and chronic UTI in Europe for over 40 years. It’s what is called a “urinary tract antiseptic.” Methenamine relies on a pH < 5.5 to be effective so urinary pH acidifying agents are prescribed along with it—namely, 1 – 4 grams per day of Vitamin C (ascorbic acid).

Formaldehyde what?!

Methenamine is broken down into ammonia and formaldehyde which is responsible for its urinary bacteriostatic effect, meaning it stops bacteria from reproducing while not necessarily killing all of them.

How is methenamine used?

It is most effective for short-term (< 7 days) use in preventing recurrent UTIs in women. In women with recurrent UTI caused by multidrug- resistant bacteria where other preventive strategies have not worked, methenamine offers a good choice.

What antibiotics should I try for UTI prevention before methenamine?

Nitrofurantoin 50 mg – 100 mg or low-dose Bactrim 40 mg/200 mg taken nightly has long been used for women with frequent recurrent infections. Nitrofurantoin is preferred as there is less than 5% resistance of E Coli to nitrofurantoin—though, that is increasing.

Can I take this when I have an acute/active UTI?

No. Methenamine is not effective for an active (current) urinary tract infection. It’s used instead for prevention if you have recurrent UTIs.

What are the upsides of methenamine?

Resistance to this antibiotic (the formaldehyde) doesn’t happen, which offers a huge advantage over conventional antibiotics and may allow for prolonged use, though that remains to be seen.

Other upsides? It does not affect gut flora (no antibiotic-associated diarrhea), is relatively non-toxic, and low cost.

Dr O.

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