Long term dxm (dextromethorphan) effects?

I have been abusing DXM (the active ingredient in cough syrup -dextromethorphan) for almost 2 years on and off. I sometimes go weeks on DXM binges constantly taking it and sometimes i just feel lifeless. Im not addicted to it as i had stopped many times but i have severe back problems and im in a lot of pain and DXM is the only thing that works for me. I just don't want to overdose but i want personal experience responses . I know it is rumored that it can cause holes in the brains and stuff like that but there is no true evidence . but i just want to know if anyone had any problems or effects after long term use