D-mannose is a type of sugar naturally found in many plants, fruits and berries, for instance in cranberries. It is also made by the body where it has important jobs in the immune system and the making of mucus.
Research suggests that most of the D-mannose you take as a supplement is passed out of your body in your pee. This D-mannose in your pee has the potential to interfere with the ability of the bad bacteria causing UTI’s (E.coli) to stick to the walls of your urinary tract. This process is called ‘competitive inhibition’ and maybe one of the ways that D-mannose helps the body fight a UTI/cystitis.
The National Institute for Care Excellence (NICE) says of D-mannose
The committee noted evidence suggesting that D-mannose was effective in reducing the risk of recurrent UTI in non-pregnant women, and noted the low NNT of 3 (range 2 to 3) over 6 months, compared with no treatment. However, this was based on 1 small RCT. The committee agreed to make a recommendation that some women who are not pregnant may wish to try D mannose, as a self care treatment, noting the sugar content of this product which should be considered.
https://www.nice.org.uk/guidance/ng112/chapter/summary-of-the-evidence#self-care-2