Experts who reviewed the research say that it has important limitations that may explain why women who took the capsules didn't see benefits.
The study recruited 221 premenopausal women with a history of frequent bladder infections and split them into two groups.
The first group got a daily dose of 480 milligrams of the antibiotic Bactrim. The second group took 1,000 milligrams of cranberry extract -- the brand used in the study was Cran-Max -- every day.
Study participants regularly took urine samples, which were tested for bacteria, and they noted any signs of UTIs, like back pain, or burning or urgency with urination.
After a year, the average number of UTIs reported by the antibiotic group was about two, compared to four in the cranberry group. Before the study started, the women said they'd averaged six to seven UTIs in the previous year.
While the group taking the antibiotics had fewer infections, they also developed much higher levels of resistance not only to the drugs they were currently taking but also to other antibiotics.
That's a concern, says study researcher Suzanne E. Geerlings, MD, PhD, of the Center for Infection and Immunity Amsterdam at Academic Medical Center in the Netherlands, because it means that doctors have fewer weapons to turn to when an infection flares.
"In my opinion, it is not time to give up cranberries," Geerlings tells WebMD in an email. "The take-home message for women with recurrent UTI is that cranberries are less effective in prevention but do not result into resistant microorganisms."
Doctors who treat women with UTIs say they like the study because it weighs in on something patients often ask: Do cranberries work, and do they work as well as antibiotics?
"I think it's a good study," says Elizabeth Kavaler, MD, a urology specialist at Lenox Hill Hospital in New York City. "The outcome is what you would expect, that the patients on the antibiotics are going to do better."
She says that while cranberries may offer some benefits, they can be tough to use as a medicine.
"There's a bunch of problems with cranberries," Kavaler says. "The pills that you get, we don't even know what's in them. Then, how much do you need?"
Doses, Study Design Questioned
Other experts say that the research has some limits that prevent it from being able to settle any questions about the effectiveness of cranberries for UTIs.
The first is that the study didn't include a comparison group of women who weren't getting any prevention at all. Comparison groups are important to give an idea of the magnitude of effect that an intervention might be having.
The second is that while women who were taking the antibiotics reported having fewer UTIs than the cranberry group did, the number of infections confirmed by laboratory testing was not statistically different between the two groups.
The third was that the cranberry capsules used in the study had relatively small amounts of active ingredients in them. "The dose of the cranberry that they used in that particular study was way too low," says Bill J. Gurley, PhD, a professor in the department of pharmaceutical sciences at the University of Arkansas for Medical Sciences, in Little Rock.
Gurley says a government-sponsored study is under way that may shed more light on how much cranberry is needed to prevent infections.
Other studies have found that when cranberry extract is used in higher doses, about eight times as high as were used in this study, they do appear to keep bacteria from sticking together in the bladder, which may help prevent an infection.
"The studies that have been well-designed, and there have been a handful that were well-designed," have shown that cranberries can be effective, says Amy B. Howell, PhD, an associate research scientist at the Marucci Center for Cranberry and Blueberry Research at Rutgers University in New Brunswick, N.J.
"If the bacteria can't bind to the bladder wall, they will not inject their toxins and grow and cause an infection, they just get washed out in the urine stream," says Howell.
"The key here, which has relevance to this latest study, is that you're not killing the bacteria. You're not building up these populations of resistant bacteria," which should keep them sensitive to antibiotics in case an infection does take hold, she says.