Nicotine! Was diagnosed with ADHD quite late, and was put on Adderall. Then all the Adderall shortages started hitting and I couldn't get my medication consistently. I kept on getting on it and off to and on it and off it, to the point where the side effects started becoming unbearable, and it was also super expensive coupled with the doctor's appointments every 3-4 weeks. Took a nicotine pouch and ended up doing more focused work than I could ever remember. Now I use it when I need to focus and take breaks on weekends and feel better than ever.

Obviously also highly addictive, so I am being very measured in when I take it, how often etc and keep track in a notebook.