I Currently Am Being Treated For Cancer, SpinaI Tumors Mostly

I currently am being treated for cancer, spinaI tumors mostly on the left side of the spine. In addition or as a result of ( I don't know which) am immobilized with back pain for which I am being treated with pain meds Percoset and Dilaudid and Gabapentin. Also PT, which is helping but very slowly. Today my therapist suggested Flexaril, which was prescribed for me a short while back when I had an arm issue. I have some in the house, Is it safe to take them? I had no side effects before.