Citrus soft drinks containing grapefruit juice can inhibit the metabolism of some drugs.
Sun Drop and Fresca sodas contain bergamottin...one of the constituents in grapefruit juice thought to be responsible for it's CYP3A4 inhibition. This has led some to believe that these soft drinks could affect metabolism of some drugs.
There is only one documented case report of this possibly occurring. A transplant patient experienced a doubling of his cyclosporine levels on two different occasions after consuming Sun Drop. And intentional avoidance of this soft drink prevented recurrence of the problem.
But this doesn't prove that the citrus soda was responsible. In fact, a follow-up study failed to show any effect of either Sun Drop or Fresca on cyclosporine levels in healthy volunteers.