You don’t always need drugs to change brain chemistry, you can change eating habits to change your mood, for example, and that is because you changed your brain chemistry. You can also re-wire your brain or utilize other areas of it to carry out functions, think of people that have had a brain injury and had to relearn how to do an activity like walking, they had to relearn because their brain had to be re-wired. Speech therapy isn’t superficial, if used right it teaches a person how to speak using a different path and if used for long enough the path can become permanent or semi-permanent. I’m 47 and I’ve used my speech fluency skills for so long that they’re second nature, I rarely speak without using them.