I took Phentermine a number of years back, in the hope it would help "kick-start" some weight loss.
I had no weight loss, but I did have a boost of energy. It acted more like a dose of speed for me, which given the fatigue from my Fibromyalgia, was a welcome side effect!
After a year or so, my doctor discontinued my use, as it is linked to some cardiac issues, & we figured for long-term, better safe than sorry.
Keep in mind, if it works as an appetite suppressant, you'll need to have a strong plan in place before discontinuing it, so that you don't overeat & gain any weight back. Definitely make plans to be successful! :)