"Off-label" means the medication is being used in a manner not specified in the FDA's approved packaging label, or insert. Every prescription drug marketed in the U.S. carries an individual, FDA-approved label. This label is a written report that provides detailed instructions regarding the approved uses and doses, which are based on the results of clinical studies that the drug maker submitted to the FDA.
“Many people may be surprised to know that the FDA regulates drug approval, not drug prescribing, and ... doctors are free to prescribe a drug for any [reason they think is medically appropriate],” says G. Caleb Alexander, MD, MS, a medical ethics advocate and assistant professor of medicine at the University of Chicago Medical Center. "Off-label use is so common, that virtually every drug is used off-label in some circumstances."