21 CFR §314.520

1. (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. (1)Distribution restricted to certain facilities or physicians with special training or experience; or
   2. (2)Distribution conditioned on the performance of specified medical procedures.
2. (b)The limitations imposed will be commensurate with the specific safety concerns presented by the drug product.