Stuttering treatment programs: do they actually work?

Stuttering treatment programs: do they actually work? Hi everyone, In my country (Italy) there are so many courses about solving stuttering. I've done some of them, spending thousands of dollars, and while the majority of participants don't have great improvements, I've met some and heard of many others that were able to solve the problem completely in around a year. The programs here are very different from each other, having in common only the fact that they all require great effort in terms of hours spent and uncomfortable actions needed for exercises. To me, it's not very clear how they all can achieve results for many people or if there's any scientific backing to their methods. Does anyone have similar experiences? Do you think it's actually possible to solve stuttering? And the most important question is: is there any scientific paper about the most effective ways to solve stuttering?