Acetametaphine symptoms and conditions

I've taken Ultracet for about 4 years almost every day, morning and night time and sometimes in the afternoon depending how severe my pain is. I take 2 pills each time. It does help take the edge off my pain. My doctor said my Fibromyalgia is as bad as it can get, plus I have severe arthritis through my whole body. I can hardly stand up straight and trouble walking. Ultracet seems to help better then Ultram for me. I'm concerned about how long I've taken Ultracet and damaging my liver or other organs. Does anyone know how long you can continue using Ultracet before it does damage. I'm very concerned about that issue because it does somewhat help with pain. I haven't noticed any side effects using Ultracet. If anyone knows how long you can stay on Ultracet without any harm, please post your knowledge or experience. Should I also have my blood monitored periodically?