Can someone tell me how to repair spinal/nerve tissue damage due to drug abuse?

You can't: "repair" it by anything you do. Some people might advise making sure you have adequate b vitamins, particularly thiamine, which is necessary for good nerve function, so you might take a therapeutic multivit, but I am unaware of any studies that would indicate they work. Nerves may heal over time. If the nerve was actually physically damaged, you could see a neurosurgeon for advice. Stop using.