D-mannose is a simple sugar that occurs naturally in the human body and is found in many fruits. It’s also available as a dietary supplement, which people often take to help prevent or treat urinary tract infections (UTIs).

Research shows that supplemental D-mannose can help treat a rare condition called carbohydrate-deficient glycoprotein syndrome (CDGS) type 1b. Preliminary studies also show it may act as a prebiotic to promote the growth of healthy gut bacteria.

D-mannose can help treat a rare genetic disorder called carbohydrate-deficient glycoprotein syndrome type 1b because people with this condition don’t produce enough, or any, mannose naturally.

CDGS often disrupts the blood clotting process, which can lead to bleeding, blood clots, or stroke-like episodes. It also leads to gastrointestinal issues, particularly protein-losing enteropathy—a syndrome where excessive proteins are lost in the gastrointestinal tract—which can cause low protein levels in the blood.

Taking D-mannose daily may help prevent some of these complications by introducing mannose to the body from an external source. 

Over 50% of adult women will get a UTI in their lifetime, making UTIs the most common type of outpatient infection.

The infection is typically treated with antibiotics, but researchers are interested in D-mannose as an alternative treatment that won’t promote the growth of antibiotic-resistant bacteria. 

UTIs are commonly caused by Escherichia coli (E. coli) entering the urinary tract. D-mannose prevents E. coli from attaching to the urinary tract as the body excretes the bacteria through your urine. Various studies show that D-mannose is as effective as antibiotics for preventing recurrent UTIs. 

Larger studies are necessary to confirm the benefits of D-mannose for UTIs.

D-mannose may also act as a prebiotic—a dietary fiber that feeds the good bacteria in your gut. Bacteria in the large intestine ferment (break down) prebiotics, helping good bacteria thrive, producing beneficial short-chain fatty acids, improving the integrity of your gut lining, and boosting immune health. 

Unlike glucose and fructose, D-mannose is a sugar that may reduce rather than increase inflammation in the gut.

In rodent-based studies, D-mannose increased the Bacteroidetes to Firmicutes ratio in the gut, which is notable because low levels of Bacteroidetes are linked with inflammatory bowel disease (IBD).

Current studies are limited to animals, so human studies are necessary to know whether D-mannose acts as a prebiotic in the human digestive tract.

D-mannose supplements are available in capsule and powder form. You may also see D-mannose combined with cranberry and marketed for urinary tract health.

Research shows D-mannose is safe to take to prevent UTIs for up to six months, but the best way to do so isn’t yet standardized.

Some studies have found benefits from taking 1.5 grams of D-mannose every day for one week each month. Other research has shown benefits from taking 3 grams split throughout the day for two weeks, followed by 2 grams daily for six months. 

Dosage 

D-mannose supplements typically come in servings of 500–2,000 milligrams. Research shows that taking 3 grams daily for up to three weeks or 2 grams daily for up to six months is generally safe. There is no standardized dosage of D-mannose.

Because of insufficient research, it’s best to avoid D-mannose while pregnant or breastfeeding. People with diabetes should also be cautious about taking D-mannose supplements, as they may alter blood sugar levels.

Otherwise, D-mannose is generally safe for adults to take orally for up to six months. It’s usually well-tolerated, aside from some digestive side effects–which you can reduce by splitting the dose throughout the day.

There isn’t enough research to know whether it’s safe to take D-mannose for longer than six months, so talk to a healthcare provider before taking it long-term. 

Potential Drug Interactions

There are no known drug or supplement interactions for D-mannose, which may be due to a lack of research. Speak with a healthcare provider to ensure it’s safe for you to start a D-mannose supplement with your current medications or supplements.

Can You Take Too Much?

No upper limit is set for how much D-mannose is safe to take. While it exists naturally in several fruits, the amount found in food is much lower than what you’ll get from a supplement.

Excess D-mannose gets excreted in the urine rather than building up in your system, which generally means the risk of toxicity is low. Exceeding 200 milligrams per kilogram of body weight at once increases the chances of diarrhea, but most supplements come in doses far lower than this.

It’s best not to exceed the dosage recommended by your healthcare provider or the supplement manufacturer.

D-mannose is unlikely to cause serious adverse effects, but some people may experience digestive side effects from the supplement. The chances of experiencing side effects are higher if you take a high dose all at once, especially if you exceed 200 milligrams per kilogram.

Potential side effects of D-mannose include:

D-mannose is a simple sugar found in fruits and available as a dietary supplement. It may help prevent or treat UTIs by blocking bacteria from sticking to your urinary tract.

D-mannose can also help manage symptoms of a rare genetic disorder called carbohydrate-deficient glycoprotein syndrome type 1b. It may also act as a prebiotic to promote the growth of healthy gut bacteria. 

In general, D-mannose is safe for healthy adults to consume as a supplement, but it may have side effects like diarrhea, bloating, or nausea. Before starting a D-mannose supplement, speak with a healthcare provider to ensure safety and appropriate dosing.