The other thing to understand about these drugs is that they only work for some people, it's a bit of a genetic lottery. They work by binding to certain receptors which happens more readily in some patients than others, this can make the pharmaceutical science a bit unreliable.
I'm one of the lucky ones in that regard, and I don't seem to have developed any resistance. Mind you, the dose I take is low, only a quarter of what is usually prescribed per day. If the drug isn't working now it's probably not going improve, which is a bugger. You can only give it a go I suppose.