@jrenjr
https://www.fda.gov/drugs/frequently-asked-questions-popular-topics/generic-drugs-questions-answers.
I have taken both brand name Revlimid and generic lenalidomide, as have several others i know. A generic must have the same active ingredient by law, but may have different fillers/colors/preservatives. A person could react to those different non-active ingredients.

How are you doing on your health journey with multiple myeloma?
Ginger