ATC in RxNorm
Can scrape DDD
RxNorm published ATCPROD which links ATC codes to Product-level RXCUIs. Only available in RxClass. No plans to incorporate into RxNorm data files per RxNorm team.
WHOCC - ATC/DDD Index
A searchable version of the complete ATC index with DDDs is available below. The search options enable you to find ATC codes and DDDs for substance name and/or ATC levels. In your search result you may choose to show or hide the text from the Guidelines for ATC classification and DDD assignment linked to the ATC level.
Example of hitting the RxClass API for all SCDs (excludes some SBDs / GPCKs)