ANSWER: It is known to have positive effect for some people but not all..
D-mannose is a type of sugar that is often used as a natural remedy for Interstitial Cystitis (UTIs). While there are claim that it can be effective in treating and preventing Interstitial Cystitis, scientific evidence to support these claims is not endorsed by all. It is always best to consult with a healthcare provider before using any natural remedy for a medical condition.
Interestingly, there is advice on NHS Website: Ways to prevent some UTIs coming back, If you keep getting a bladder infection (cystitis), there is some evidence it may be helpful to take: D-mannose – a sugar you can buy as a powder or tablets to take every day
See The NHS article at https://www.nhs.uk/conditions/urinary-tract-infections-utis/