It is a horrible horrible idea to use phentermine for fat loss purposes - there are much healthier and more effective alternatives, so better stay away from this shit. It can cause way too many stim related side effects, which will stress your body way too much. Besides, even if you will loose fat with it, the results won't be long lasting, and you will easily get back all the weight once you stop using these potent stims.
Instead, better consider using such things as Cardarine GW or SR9009 - they are not stims, non-hormonal, and they will cause no side effects at all. However, they will make you lose fat effectively, while maitaining your lean mass, and the results will be really long lasting.