The FDA's published file of every drug currently or recently listed, updated daily and distributed through openFDA.
The directory contains finished and unfinished drugs, prescription and over-the-counter products, and both marketed and recently delisted entries. Presence in the directory means a product was listed, not that it is approved or currently available.
Records disappear from the current file once listing expires, which is why a valid historical NDC on an old claim may return nothing today.