The extract comes from the Nerium oleander plant.
The FDA's response letter, dated August 14, was posted by the agency on Wednesday. It said the company had already tested oleandrin as a potential prescription drug and could not at the same time seek permission to sell it as a supplement -- a category with almost no oversight.
Even if it hadn't been excluded from the definition of a dietary supplement under the law, the agency had "significant concerns about the evidence included in submission.