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You are suggesting a viable, commonsense procedure. The problem is that the NFL is not noted for implementing such reasonable solutionsThe supplements can have the formula changed after the testing the player submits it for. (that is what happened in the Starcaps case).
I have often wondered why the NFL doesn't start a program where a manufacturer submits their products for testing and if it passes they are allowed to put a "approved by the NFL" on the bottle. The league would also at random times pull random samples off a store shelf and test to see if they have changed it since they submitted for approval. If they have then not only that product but all products the company produces become banned and word is put out to all players through the NFLPA to immediately stop using the companies products. The cost of the program would easily be covered by licensing fees and any profits after expenses split between the NFL and NFLPA.