Valium AND Prozac

Does anyone know if someone can become immune to the effects of either of these drugs?

I have taken 5mg valium for anxiety on an as and when basis for a few years, now when i take one it doesnt take the edge off anymore. Been on prozac for 5 months and its great but i am finding im having break through SOB attacks again, Does anyone think the body becomes used to a dosage and therefore might need more and more higher dosages.