21 C.F.R. § 314.520 : Food and Drugs — FOOD AND DRUG ADMINISTRATION, DEPARTMENT OF HEALTH AND HUMAN SERVICES — DRUGS FOR HUMAN USE — APPLICATIONS FOR FDA APPROVAL TO MARKET A NEW DRUG — ACCELERATED APPROVAL OF NEW DRUGS FOR SERIOUS OR LIFE-THREATENING ILLNESSES — Approval with restrictions to assure safe use

(a) If FDA concludes that a drug product shown to be effective can be safely used only if distribution or use is restricted, FDA will require such postmarketing restrictions as are needed to assure safe use of the drug product, such as:
(1) Distribution restricted to certain facilities or physicians with special training or experience; or
(2) Distribution conditioned on the performance of specified medical procedures.
(b) The limitations imposed will be commensurate with the specific safety concerns presented by the drug product.

21 C.F.R. §314.520