Grapefruit is known to interact with many medications. According to MayoClinic.com, grapefruit interacts with anti-anxiety, anti-arrythmia, antidepressant, antihistamine and certain blood pressure medications. This occurs because certain enzymes used to metabolize grapefruit also act on certain medications, which results in a higher concentration of medication in your bloodstream. However, grapefruit has not been shown to increase the concentration of of the calcium channel blocker Norvasc, also known by its generic name amlodipine, in the bloodstream.
Effect of Grapefruit on Liver Enzymes
Grapefruit is an inhibitor of the cytochrome P450 system, according to a 2006 article published in the journal "Clinical Pharmacology and Therapeutics." Specifically, grapefruit inhibits the actions of the CYP3A4 pathway. Therefore, it will cause an increase in the concentration of substances and drugs metabolized via this pathway. Therefore, there will be an increased effect of these drugs on the body, which may require a decrease in dosage to prevent side effects or overdose.
According to a 2000 study in the "British Journal of Clinical Pharmacology," Norvasc is liver metabolized, specifically by the CYP3A4 pathway. Because grapefruit is also metabolized by this similar pathway, theoretically, its metabolism should be affected when you eat grapefruit. However, the researchers found no effect on the metabolism of Norvasc with one serving of grapefruit juice per day. This finding is in contrast to other medications similar to Norvasc.
Different Metabolic Pathways
The researchers offered possible explanations for the lack of effect of grapefruit on the metabolism of Norvasc. First, they stated that the human CYP3A liver enzyme family is composed of closely related genes, which may explain why certain medications metabolized by the CYP3A system are affected by the consumption of grapefruit juice while others are not. In other words, there may be another subset of the CYP3A family involved in Norvasc metabolism that grapefruit juice does not affect.
In addition to inhibiting the CYP3A pathway, grapefruit juice also inhibits another liver transport molecule called P-glycoprotein. Medications that need P-glycoprotein transport into cells are increased in the bloodstream because P-glycoprotein becomes inactive after drinking grapefruit juice. Researchers state that there is no evidence concluding that P-glycoprotein is involved in Norvasc metabolism, which may explain its lack of effect on Norvasc pharmacology.