I was diagnoises with struvite stones(kidney), formed by a chronic infection. The drug they gave me was methenamine, what it does when it comes into contact with acid urine, it produces formaldhyde. Now, knowing that formaldyde is a posion, this really scares me. Will it hurt my kidney's? Is it a safe drug? PLEASE, if anyone has any info I sure would appreciate it so very much. she wants me on it long term. When I talked to the nurse about my fear, she agreed, but said it was the only drug that will help, and said she too would be nervous.