Yeah, although up to 15 years sounds great, I'd be a little hesitant to go past 3 years. I've taken many prescription drugs (muscle relaxants, pain killers, sleep meds) up to 2-2.5 years past and they all seemed to work just fine. Heck, I still have a bottle of about 30 Skelaxin still left over from over 2 years ago that I still use when my back starts acting up. I'm prescribed Flexeril but Skelaxin just works much better for me.