Flexeril and maybe another muscle relaxer aren't known to be addictive (unless you mean that people rely on them, but if you need it to control spasms, then you need it). I believe Soma is the one you're thinking of, as it can be habit-forming.

Do you have a source where you found that flexeril can cause a relapse, H? I can't find any good info with a basic search.

28 y/o HLA-B27 neg
Dx: AS/SpA, cardiac arrythmia, Hashimoto's
Currently taking: remicade, flexeril or nucynta ER occasionally