In transcript of this video, Dr Greger mentions that he nearly poisoned himself by eating lots of raw elderberries picked from his backyard.
Dr Greger explains that elderberries must be consumed after cooking, not raw. CDC Report on toxicity due to raw elderberry juice. A personal narrative here (by Elderberry Edge Farm owners).
(5 mins) Transcript. Dr Greger's summary: "How effective are flu shots, elderberries, echinacea, and cranberries?"