Off-label use of all drugs is a very common practice and is entirely legitimate and legal. About one in every five prescriptions written in the US is for off-label use, according to the Journal of the American Academy of Psychiatry and the Law (JAAPL).
Ultimately, your doctor is the person who decides what is best for you and can prescribe any drug, whether or not it is labeled as intended for that specific use by the FDA. This is stated clearly on the FDA’s own website here.
“Once the FDA approves a drug, healthcare providers generally may prescribe the drug for an unapproved use when they judge that it is medically appropriate for their patient.… One reason is that there might not be an approved drug to treat your disease or medical condition.  Another is that you may have tried all approved treatments without seeing any benefits.”