The art of off-label prescribing
Some drugs are used for indications beyond their original FDA approved ones that make complete sense. I mean, if old seizure medications help nerve pain, it might be reasonable to try new ones for the same purpose if everything else fails.
Sometimes the broader use of a medication leads to additional FDA approved indications. One good example is bupropion, Wellbutrin, for smoking cessation. It even got a new name for that indication, Zyban, …