I've taken Zoloft for years...like maybe 10 or so. Worked very well, then several months ago I realized I was feeling like a zombie and just had no desire for anything.

I told my pdoc I thought Zoloft had lost its efficacy on me and he prescribed Cymbalta. (Now I'm having issues with that but it's on another thread.)

So, my opinion is that a med can definitely either stop working or somehow change over time...unfortunately. *sighs*