I have suffered several UTIs. For me, D-Mannose did end up clearing up the UTI but at a price...I had horrible abdominal pains because of the gas buildups. I guess this is sort of a freak reaction that happens to some people, because my friend took it (as do many others based off of reading the reviews), and they don't seem to have this issue. I took the recommended dosage on the bottles. I got two different brands just to make sure one brand wasn't bad, but had the same reaction with both.
I have since switched to colloidal silver for treatment for UTIs and it works wonderfully, and is faster and more effective than all other treatments I have used. Also, as a sidenote, I have gone over 7 months without a UTI since I started to change my diet. Generally, I eat little to no meat or dairy, and lots of veggies, fruits, and whole grains. Though meat for me seems to be more of a culprit than dairy. If I eat meat more than 3 days in a row, I tend to get a UTI. It's crazy.