This is a comprehensive article on how grapefruits and grapefruit juice can have a dramatic/adverse effect on medications. Let your doctor know if you consume it regularly (same goes for Brussels sprouts/cabbage, which have different effects on medications). Click here for the full article.
It's also important to tell your doctor which supplements/over-the-counter products you take, as they could affect the absorption of various meds and/or interact dangerously with prescribed medications.