An optiate is not a treatment for any myositis disease. If you suspect you have polymyositis or dermatomyositis you should talk to your rheumy about that and get the appropriate testing. CPK, Sed, aldolase...emg, and a muscle biopsy if there is sufficient evidence that you have it. Treatment is usually prednisone at first with other pred sparing meds added so pred can be tapered.
Treating the symptom (pain) will have no impact other than masking the problem. Some myositis patients have muscle pain but some do not. I had terrible pain for the first few months. Pain and weakness are related but you have to be sure it is muscle pain. As the muscles weaken there is significant joint pain because they are not supported properly. Myositis can cause permanent or temporary muscle damage so it should never be untreated.
If your mother's weakness was more in her legs and distal muscles and it progressed very slowly over the course of years it could have been inclusion body myositis which is an entirely different disease than pm or dm. The muscle weakness in pm or dm is usually symmetrical and affects the proximal muscles more.
MCTD (lupus, scleroderma, polymyositis). Diagnosed 2005. Kidney, liver, GI tract, dysphagia, raynauds, Barretts esophagus, quadriplegic in 2005. Recovered and now active in skiing, tai, chi, hiking, golf. Meds: prednisone 2.5mg, imuran 25mg, amlodipine, benazapril, omeprazole, potassium, folic acid, vitamins, maxide and various supplements and vitamins.
Remain optimistic and you can overcome the odds.