Cranberry actually works, a little. A 2023 review of 50 studies and a fresh 2025 trial both show cranberry cuts the chance of getting another UTI in women who get them often. The 2025 medical guidelines now say doctors may offer it.

The dose matters more than the brand. Look for a capsule or extract that gives you at least 36 mg of A type PAC twice per day (PAC is the active ingredient in cranberries). Most juices don’t give you enough.

The 2024 SWEET trial flipped the D mannose story. In a study of 598 women, taking 2 grams of D mannose a day was no better than a sugar pill. Same number of UTIs. Same number of antibiotic visits.

Vitamin C rests on one small study. Only one trial has ever tested vitamin C for UTI prevention, and it was done in 110 pregnant women. There’s no study in non pregnant women, and current guidelines don’t recommend it on its own.

Don’t go over 1,000 mg of vitamin C a day. That much vitamin C roughly doubles the risk of kidney stones in men. Lower doses are fine.

Want to know the science? Keep reading below 👇

As a urologist who takes care of women with repeat urinary tract infections, three supplements come up at almost every visit: cranberry, D mannose, and vitamin C. Patients walk in with the bottles already in their bag, usually after years of advice from family, friends, or the internet.

In 2024 and 2025, the science on all three changed enough to take a fresh look. One supplement is better supported than most people think. One just failed its biggest test. The third is built on a good idea and almost no evidence.

Most simple UTIs in women are caused by E. coli, a kind of bacteria that lives in the gut and sometimes makes its way to the bladder. Once it gets there, E. coli uses tiny hairs and hooks on its surface to grab onto the cells lining the bladder. If it can’t grab on, it gets washed out the next time you pee.

Each of these three supplements goes after a different step in that process. Cranberry blocks one type of hook. D mannose is supposed to plug the other type of hook. Vitamin C is supposed to make your urine more acidic, which bacteria don’t love. The ideas all make sense on paper. The real world results are very different.

The most careful study of cranberry is a 2023 review that pooled 50 different trials and 8,857 people. It found that cranberry products probably lower the chance of a UTI by about 26% in women who get them often. That’s not a miracle, but it’s real.

A separate 2025 trial gave 150 women with repeat UTIs a daily 500 mg capsule of cranberry powder or a placebo. The cranberry group had 52% fewer confirmed UTIs, and the ones who did get a UTI got it later.

A 2025 analysis that pooled even more studies came to the same conclusion: cranberry helps. The 2025 U.S. and Canadian urology guidelines now say doctors “may offer” cranberry to women with recurrent UTIs. It is also on the American Geriatrics Society’s list of non antibiotic alternatives.

The active ingredient in cranberries is a group of molecules called A type proanthocyanidins, or “PAC” for short. Lab studies suggest you need at least 36 mg of PAC twice a day or 72mg once a day for cranberry to do anything useful. Higher doses don’t seem to work better in real trials, so chasing huge doses is not the move.

Capsules and standardized extracts usually beat juice. Juice products are all over the map on PAC content, often loaded with sugar, and easy to under dose. If a patient really wants juice, I ask them to look at the label and tell me how much PAC is in one serving. Almost no one can.

Cranberry is generally safe. The most common side effects are stomach upset, nausea, and loose stools, and even those are not much worse than placebo.

The one interaction I always flag is warfarin, a blood thinner. Very high doses can push warfarin levels too far and raise bleeding risk. If you are on warfarin, stick to standardized extracts at modest doses and have your INR (the blood thinner blood test) checked after you start.

D mannose is a kind of sugar that is supposed to plug a hook on E. coli called FimH, so the bacteria cannot grab onto the bladder. Early studies looked great. The most famous one (Kranjčec, 2014) reported that women on D mannose had UTIs only 14.6% of the time compared to 60% in the comparison group.

The problem: that study was not blinded. Patients knew what they were taking, and so did the researchers. That is a setup that often makes treatments look better than they are.

Then came SWEET (2024). It was the biggest and best designed test of D mannose yet: 598 women with repeat UTIs in UK family doctor offices, randomly given either 2 grams of D mannose a day or a placebo. Neither the patients nor the doctors knew which one they were getting.

The results:

If you read a 2025 paper by Han and colleagues that pooled 50 trials, D mannose comes out on top as the most effective non antibiotic option. That seems to contradict SWEET.

It does not, really. That analysis leans heavily on the older, unblinded studies, the same ones the authors themselves flagged as low quality. SWEET was specifically designed to settle the question, and it did.

Bottom line on D mannose: the best designed trial says it does not work. I no longer routinely recommend it. It is still safe (the main side effect is loose stools), so if a patient really wants to keep taking it, I do not push them to stop.

The idea is that vitamin C makes your urine more acidic, which is supposed to be a tougher environment for bacteria. In practice, it is unclear how much oral vitamin C actually changes urinary pH or whether small changes matter.

The only trial that has ever tested vitamin C just for UTI prevention is Ochoa Brust, 2007. It included 110 pregnant women in Mexico. Half got iron and folic acid; the other half got iron, folic acid, and 100 mg of vitamin C a day for 3 months. The vitamin C group had fewer UTIs (about 13% vs. 29%).

The catch: this study had serious quality problems. Patients were assigned by whether their chart had an even or odd number (not real randomization), there was no blinding, no placebo, and the dropouts were not tracked well. A Cochrane review of vitamin C in pregnancy rated it as high risk of bias. A separate review of UTI prevention in pregnancy said the evidence is too weak to recommend.

There is no study at all in non pregnant women, and the 2025 U.S. and Canadian urology guidelines do not recommend vitamin C on its own.

The recommended daily amount of vitamin C is 75 to 90 mg. Most cranberry plus vitamin C combo pills stay well under the safety ceiling and are fine.

The concern is standalone high dose vitamin C. In two large U.S. studies that followed nurses and male health professionals for years, men who took 1,000 mg or more a day had roughly twice the rate of kidney stones. The reason: extra vitamin C gets turned into oxalate, and oxalate forms stones. Risk in women is more mixed, but the same chemistry applies. My rule of thumb: stay under 1,000 mg a day from all sources. Under 500 mg looks safe.

Cranberry supplements, Vitamin C and D-mannose supplements have not been shown to help UTI prevention in:

Cranberry works. A 2023 Cochrane review, a 2025 trial, and a 2025 pooled analysis all point the same direction. Aim for at least 36 mg of A type PAC a day from a capsule or extract.

D mannose does not. The 2024 SWEET trial, the biggest and best designed one we have, found no benefit over placebo. Older positive studies were unblinded.

Vitamin C is a side note. One small, low quality pregnancy trial is not enough to recommend it on its own. Keep total intake under 1,000 mg a day.

If supplements alone are not enough, especially for women after menopause, the next step is a conversation with your doctor about vaginal estrogen, methenamine, or targeted antibiotics.

If you know a mom, sister, partner, or friend dealing with repeat UTIs, please share this. The supplement aisle is loud. The evidence is quieter, but it points somewhere useful.

Last week: Vaginal Estrogen for Recurrent UTIs: A Urologist Explains 30 Years of Evidence, Yet Still Underused. Why a 1993 trial still gets ignored, and what women after menopause actually get out of low dose vaginal estrogen.

Coming next in the supplements series:

If you are curious about omega 3s and fish oil, here are the prior Pareto Life issues in the fish oil series:

Williams G et al., Cochrane 2023 · Stonehouse W et al., Am J Clin Nutr 2025 · Han Z et al., Infection 2025 · Ackerman AL et al., J Urol 2025 · Anger JT et al., J Urol 2022 · Hayward G et al., JAMA Intern Med 2024 (SWEET) · Kranjčec B et al., World J Urol 2014 · Ochoa Brust GJ et al., Acta Obstet Gynecol Scand 2007 · Steinman MA et al., J Am Geriatr Soc 2025 (AGS Beers) · Talasaz AH et al., Semin Thromb Hemost 2025 · Madden E et al., Planta Med 2021 · Ferraro PM, Curhan GC et al., Am J Kidney Dis 2016 (NHS or HPFS).