What am i doing wrong? I started taking phentermine 4 days ago. I lost 8 pounds in the first two days. I've been eating healthy and in smaller portions. And I've been working out. The 3rd day, I gained 3 pounds, and today I gained back 5. It's really discouraging.. I haven't changed my meals and workouts at all. I'm not understanding why I'm gaining it all back... Ive had this issue before phentermine. No matter how healthy i ate and how much i worked out, i would just gain weight. Its not an issue with my thyroid, my doctor tested me for that. I don't know what else it could be. Maybe I'm just meant to be fat.