Elizabeth,
Very likely because of grapefruit juice and calcium-channel blocker interaction – http://www.sciencedirect.com/science/article/pii/S0895706105013051. Now, there can be a difference between a few drops of something like Biocidin and drinking straight Grapefruit Juice. The chances of a problem with Biocidin are probably quite low.
Dr. Woeller
P.S. I would have the patient share the Biocidin information with their prescribing doctor and see if there is a real concern.