Do you mean you are 9 pounds heavier than when you started, or 2 pounds heavier because you gained back what you lost? You would have had to eat over 30,000 calories more than you burned to gain that much fat back, so it's probably water (because water is mostly what you lose at first on any diet-It's also impossible to lose 5 pounds of fat in a week).
There isn't actually anything in phentermine that speeds up your metabolism. It has a neurological effect that is supposed to make you feel full all the time. The problem is, if for any reason your calorie intake gets below about 1200 calories for more than a few days, you are actually doing more harm than good. You cause your body to go into starvation mode, where you not only NOT lose weight, but your body tries to store anything you do eat.
Phentermine doesn't damage your metabolism. Not eating enough does. The purpose of any diet or pill is to keep us from eating the 2000 or more calories a day that got us overweight-not to keep you from eating anything!
Phentermine does have a "tolerance" level-most doctors won't prescribe it for more than 12 weeks without a break. If you are going to take it, or any other prescription for weight loss, you should devise a plan that incorporates healthy eating. In other words, if you are only eating 600 calories a day, you are going to have to make youself eat!
The other problem is, even if you successfully lose weight with a diet pill, it's still easy to put the weight back on-not because of the pill, but because weight loss' permanence depends on continuing, let's say it again -healthy eating! I know too many people who lost weight, thought they were done, went back to too many calories or junk, and then wondered why they gained it all back.
The "speedy" feeling some people report is a result of the chemical interference in the brain-the "energy" feeling comes along with the "full" feeling, both of which are falsified in the body chemically. You do not actually burn any more calories with it than without it.
I still think they are helpful for cravings for alot of people, but no one NEEDS a diet pill to lose weight. With or without one, you are still going to have to eat healthy and exercise to lose weight.