My Back Goes Out Or Has Major Muscle Spasms Once

My back goes out or has major muscle spasms once every other or three years or so. The last time was early 2017 my doc gave me Flexerill for ten days. We also use this medication for this over the last 20 yrs plus and I usually don't even need 10 days worth. My back has just done this again to me after I reached for some food on shelf above me. Back is now grabbing and spasing. I have leftover Flexerill but it was a year old on 4/28/18 and so to now its now near 19 months old in age. My question is the medication still affective or has it lost some of its strength because of its age. Thanks for advice