Here's a link from Cleaveland Clinic https://my.clevelandclinic.org/health/drugs/19603-methenamine-hippurate-tablets

My urologist PA prescribed it & had me add two D-Mannose capsules per day (a.m. & p.m). The normal dose of methenamine are two tablets per day (a.m. & p.m.). I used both of these meds for three years, until my kidney eGFR labs began to go down. At that point, I found that methenamine can be hard on kidneys, so we cut my dose to one D-Mannose per day (a.m) and half a tablet of methenamine per day (p.m.). So far, no UTIs and no interstitial pain with this level of meds. I hope you get relief.