Grapefruit juice is known to interact with many different types of medications, from anti-anxiety drugs to statins. Specifically, grapefruit juice inhibits the CYP3A4 system of the cytochrome P45o liver enzymes, which metabolize many ingested foods and substances. Because grapefruit juice inhibits this pathway, the concentration of these substances increases in the blood stream.
Norvasc, and other calcium-channel blockers is metabolized via this similar CYP3A4 pathway. Therefore, one would theorize that grapefruit juice would cause an increase in the effects of Norvasc, and possibly cause dangerously low blood pressure.
However, a November 2000 article in the British Journal of Clinical Pharmacology did not show this effect. Specifically, one glass of
240 ml of grapefruit juice for breakfast did not affect the pharmacology of Norvasc 10mg daily administration. Vincent et al. hypothesize reasons why Norvasc is not effected while other calcium-channel blockers are. They comment that grapefruit juice may inhibit a different subset of CYP3A enzymes. In addition, grapefruit juice inhibits the P-glycoprotein transporter, which is not needed for Norvasc metabolism. Therefore, drinking a reasonable amount of grapefruit juice one per day will not likely affect Norvasc.
From a 2002 Letter to the Editor in the British Journal of Clinical Pharmacology:
In our opinion, considering the individual variation in the pharmacokinetics of amlodipine it might be concluded that the possible interaction between grapefruit juice and amlodipine cannot be neglected in the clinical setting even though this interaction does not seem to be of great clinical significance in studies performed on healthy volunteers.