phentermine is an amphetamine derivative with a pretty long half-life (25h). i'm surprised that you can still get a script for that...
mixing stimulants and mdma is not recommended as it can increase the risks for dehydration and overheating as well as the strain on the cardiovascular system. the side effects and comedown can also be more pronounced. some people also report reduced effects with such combinations (definitely with cocaine, but also amphetamine and phentermine isn't that different from the latter).
still, many people combine amphetamines and mdma regularly. so the combination isn't life threatening or necessarily harmful, but why increase the risks when you don't have to - especially when there's a chance that the effects won't be quite as good. of course the doses of both drugs will also play a role...
i'd recommend not taking the phentermine for at least 50 hours before doing mdma.