Gaining weight on medication

So i’ve been on medication for the past year or so (dexamphetamine 5-15mg/day) and I’ve noticed something strange.

I used to have so much energy that I’d just get up and go for a run because I was excited about something. If something cool would happen, I’d just need to get up and go for a walk.

People always asked me “how do you eat so much fatty food and not gain any weight” and I’d always say “I think I just have high metabolism”. I thought it was just normal to need to go for a short walk every hour.

Now that I’m medicated, I realised I’m actually gaining weight and I think it’s because I don’t do that thing anymore. I sit comfortably and still at my desk all day and do my job. I walk to and from the train station in the morning on my way to work, and that’s it.

My work performance has definitely improved but I don’t wanna get fat. Before, I was just naturally in shape from having too much energy. My doctor is always worried about me not eating enough when I’m on meds but I think I’m having the opposite problem. Has this happened to anyone else?