I have already found the following resources and found a lot of useful information:

https://www.fda.gov/Drugs/DevelopmentApprovalProcess/HowDrugsareDevelopedandApproved/default.htm

https://www.fda.gov/Drugs/DevelopmentApprovalProcess/DrugInnovation/ucm534863.htm

https://www.fda.gov/Drugs/DevelopmentApprovalProcess/

I am interested in examining why some natural drugs are able to gain FDA approval and why others cannot. Also, I would like to know at which stage specifically those drugs fail.

Along the same lines, if anyone can provide information about an eczema drugs as a case study, that would be great. For example, an eczema drug that failed in the approval process or an eczema drug that was able to gain FDA approval.