Phentermine is probably one of the older diet pills out there. Most insurances don't cover them, and this is one of the cheaper ones, you can get a months supply for about 30bucks from any given retail chain.
Phentermine works by decreasing your appetite, you eat less, therefore you lose the weight. You have to up your protein alot though, you must eat protein bars or drink protein shakes or else you will wear yourself out and get really sick. It's not for extended use, you use it for a few months max, to get your weight down then you keep it under control
remember it's not what you eat, its how much of it you eat, and how much you exercise. If you increase your exercise double what you are doing right now, and try to cut down portions, I guarantee you will notice a decrease in weight within a month. Remember sometimes you will build muscle & muscle weighs more than fat.
I have a friend on phen & she's only lost a few pounds in the last 3-4 weeks, and she's not even that much overweight.
I'd say try the exercise and diet hard core first, then go to the drugs.