Eating grapefruit after taking certain drugs can let more of the drug enter the blood than is prescribed OR it can block the enzyme and make certain drugs less effective.  These drugs include Zocor, Lipitor, Procardia, Adalat CC, Sandimmune, Neoral, Buspirone, Entocort EC, Uceris, Pacerone, Nexterone, and Allegra.

Furanocoumarin interfere with liver and intestinal enzyme CYP3A4 to produce the grapefruit effect.  Therefore, plants containing furanocoumarin may also interact with the above drugs.