I built RxCheck as a replacement for the NLM Drug Interaction API which was discontinued in January 2024. It checks for interactions between any two drugs and returns severity, mechanism, and sources as JSON. It uses the DDInter 2.0 and the ONC high-priority list with RxNorm support. This is so all brand names, generics, and identifiers are still recognized. The free tier is 100 queries per month, no credit card required.