I've taken phentermine a few times over the past 13 or so years. The first time it worked great! I lost the weight and remember feeling energetic, the kind of energetic that we should always feel. It was a good feeling, not the kind of energy you get from sugar or sodas. Anyway, some 10 years later I tried it again and it worked for acouple of months but then stopped. Increasing the amount did not control my appetite but did give me eye and head twitchs.
I used phentermine and Adipex which is just the higher dosage. If you have never used these medications, I think they do work for a few months, but if you've regained weight and are thinking of trying the pills again, don't expect them to work the way they originally did.