The national drug classification (NDC) class category is a code used to identify each of 20 major classes to which the drug entry may belong, adapted from “Standard Drug Classifications” in the National Drug Code (NDC) Directory, 1995. The two-digit categories are general and represent all subcategories (e.g., antimicrobial agents), and the specific four-digit categories represent the breakouts of the general category (e.g., Penicillin). The general two-digit codes will include medications that do not fit into any of the subcategories (four-digit codes). Starting in 1995, the NDC four-digit classes were changed to include more classes than the previous classification in 1985. Therefore, some drugs switched from a general four-digit class into a more specific four-digit class. Additionally, drugs may be approved for several different therapeutic classes. Some drugs receive approval for additional therapeutic uses after their initial approval; thus, the same drug can change classes because of new uses.