One of the most studied of these interactions is the inhibition of CYP3A4 enzymes caused by grapefruit or its juice. CYP3A4 enzymes are responsible for the metabolism of more than 60 percent of orally-administered drugs. Drugs that interact with grapefruit include anti-infectives, anti-inflammatories, cardiovascular agents, central nervous system agents, estrogens, gastrointestinal agents, Histamine H1 antagonists, immunosuppressives, and erectile dysfunction drugs. Dental patients in particular should be aware of interactions with the sedatives triazolam, midazolam and diazepam which could cause excessive sedation.

As little as 6.0 oz of grapefruit juice may lower the amount of a drug needed to produce the desired effect, which could cause an overdose. The blood that absorbs nutrients passes through the liver before reaching the general circulatory system (the first-pass). The ability of a drug to successfully pass from the GI tract to the plasma is called its bioavailability. Grapefruit juice inhibits first-pass drug metabolism, increasing bioavailability.

Many elderly patients vacation or spend winters in southern states such as Florida, where they may be more likely to consume grapefruit and other fruits that may interact with prescribed medicines. The components of grapefruit juice believed to be clinically active are also found in limes, pumellos, and Seville oranges. Natural food products, citrus products and cabernet sauvignon wine are also known to interact with drugs. This interaction can increase the concentration of drugs in the bloodstream and enhance their potency, which can result in toxicity.

While most people know to ask their pharmacists about possible interactions between medicines, many don't realize they should also be asking about the foods in their refrigerator, according to Dennis Flanagan, DDS, MAGD. He advises that patients use caution because there are probably more food and beverage interactions that have not yet been discovered. There are ways to protect against food/drug interactions. Patients should refrain from grapefruit consumption for 24 to 48 hours before and during drug therapy, says Dr. Flanagan. He also suggests that "Patient[s] should read the drug information flyer provided in every prescribed drug and question their pharmacist. [They] should also be advised to report any unusual drug effect."