When are epidural injections good for back disc hernation pain?

Rarely. There is controversy on this. The neurology literature shows that it is rarely helpful, and then for a very limited time. There are potential complications. Therefore I do not recommend using it to treat disc herniation pain. Orthopedists generally have a different opinion about this topic, and use this treatment more. Bed rest and decompressing the disc help, unless there is motor weakness.