Brain damage after over dose. Opinion?

The end of January my sister over dosed on Seroquel and was in a forced coma for 5 days because she was violent. when she came home, the MRI scans showed no signs of damage but we were told there is still a possibility that she could have it. she improved 2 weeks after being home and staying home. Now, in April she literately has all the signs of Xanax and heroin addiction but i cant seem to find a lot of truthful information out there about brain damage showing up after this long of not having any. she is trying to tell me that her impaired speech and off balance is due to brain damage from the over dose yet these symptoms of the brain damage hasn't showed up until now. i honestly need a professional opinion. She went to a hospital almost 2 months ago and they gave her a cat scan AND mri and they didnt see anything. could there still be damage? would it show up after this long? what is your opinion?