Off-Label Prescribing is prescribing medications for conditions other than those for which they have been FDA-approved. This is a common practice.
We often discuss Off-Label Prescribing when discussing Migraine medications. Although there are over 100 medications in use for Migraine and headache prevention, only four are FDA approved for Migraine prevention. All medications used for Migraine and headache prevention were originally developed to treat other conditions, then found to be helpful for Migraine and headache also.
You can read more about Migraine medications and Off-Label Prescribing in: