I had a very similar experience and was going to post about it before seeing this post. I've stuttered my whole life and decided to take try L-theanine while I had to do a bunch of presentations for my Master's thesis. It was like night and day. I would stutter so badly during my presentations before and then suddenly my stuttering was almost gone. I bought it in powder form so that I could adjust how much I take. I would probably take something close to 400mg before my exams. In doing research on how this works (I've done my master's in a biology-related field), I've found that there is little to no chance of side effects or tolerance. L-theanine is basically an amino acid so it is not degraded in a way that would case tolerance or side effects like other biological molecules. I would highly suggest everyone who stutters to try it out and try higher doses if it doesn't work. Also try powder form as if you let it dissolve in your mouth, it will make its way through blood vessels in your mouth to get to your brain faster. If you take a pill, you might lose some of that content. Let me know what you're experiences are!