It is estimated that 15 percent of the antibiotics used by humans worldwide are taken to treat urinary tract infections (UTIs). These annoying and often painful conditions are not serious in themselves, and may clear without medication. Yet there is a risk of severe complications if a UTI spreads from your lower urinary tract – the urethra and bladder – up to the kidneys. For this reason, despite the frankly terrifying specter of antibiotic resistance that is being caused, in part, by over-prescription, many UTIs are still treated with courses of antibiotic drugs.

Though men and women both get UTIs, a decent chunk of the antibiotics prescribed are required by women prone to recurrent infections. Hoping to curb these women’s reliance on antibiotics, doctors will often recommend drinking large quantities of water, based on the theory that diluting the bacteria and flushing them out with more frequent urination will help keep UTIs from taking hold. However, the peer-reviewed science backing this simple remedy has been scant and unconvincing – until now.

A new study, led by French and American physicians and published in JAMA Internal Medicine, presents compelling early evidence that recurrent UTIs can be avoided by regularly drinking lots of water.

“For decades, it has been said that increasing fluid intake could help prevent or cure urinary tract infections,” Deborah Grady, a physician from the University of California, San Francisco who was not involved in the research, wrote in an accompanying editorial. “In this issue of JAMA Internal Medicine, a randomized clinical trial confirms that folk wisdom.”

Through a network of physicians in Sofia, Bulgaria, the team enrolled 140 premenopausal women who had experienced at least three episodes of cystitis (bladder inflammation normally caused by a bacterial infection) in the past year. All participants had to be in good overall health, have no current UTI symptoms, and drink less than 1.5 liters (0.3 gallons) of fluid per day. Half of the group was told to drink 1.5 liters of water every day on top of their regular liquid consumption, and the other half were not told to modify how much they drank. Because this study was sponsored by a bottled water company, the women assigned extra water had crates of water bottles delivered to their houses every two weeks.

Urine volume and concentration were assessed via lab tests at six and 12 months, and fluid intake was recorded each month using a fluid intake diary. Urine samples were also taken whenever a UTI was suspected.

Of the 54 women in the extra-water group who completed the 12-month study without any major deviations from the protocol, 93 percent experienced two or fewer confirmed UTIs. In contrast, 88 percent of the 59 final control participants had three or more UTIs during the study period.

Despite the inherent limitations of the trial, Dr Grady believes the results should be sufficient to encourage women with recurrent UTIs to drink more water, particularly since the quantity in question would have no ill effects other than making one need to pee quite often.

“We realize that this trial was not blinded, the primary outcome was self-reported, and it was sponsored by Danone Research, which sells the bottled water used in this study," she said. "However, the research question is important and the intervention was safe, easy, and effective (and it would be impossible to blind a trial in which drinking water is the intervention).”

But to reduce costs and wasteful plastic use, Grady recommends tap water over bottled.