FDA defines off label use as, "Use for indication, dosage form, dose regimen, population or other use parameter not mentioned in the approved labeling."
"FDA recognizes that off label use of drugs by prescribers is often appropriate and may receive endorsement from published literature; they may also receive recommendations as 'Other standard uses - i.e., recommended by subspecialty societies, CDC, etc.'" Source: FDA.
"An unlabeled use [off-label use] of a drug is a use that is not included as an indication on the drug's label as approved by the FDA." Source: Medicare.
Once a new prescription drug has been approved by the FDA, and prescriptions have been written and used by patients, patients and professionals may discover that there are additional symptoms or conditions the drug may treat effectively that were not among the reasons the drug was originally approved.
Knowing about these different ways the drug may be effective, a doctor may decide to prescribe the drug for one of these alternative reasons. Such a prescription is considered to be "off-label." The drug does not need to go through the FDA process for the off-label prescription.
In particular, generic drugs that have been in use for many years may be discovered to have alternative uses.
In the US, it is legal for doctors to write off-label prescriptions. However, it is not legal for pharmaceutical companies to promote any drugs they manufacture for off-label use. They may promote the use of drugs only for the reasons the drug was approved.
Article: "Guidelines for Off-label Communications.