American Dental Association, June 18, 2012 – Grapefruit juice can be part of a healthful diet—most of the time. It has vitamin C and potassium, substances your body needs to work properly.

But it isn’t good for you when it affects the way your medicines work. Grapefruit juice and fresh grapefruit can interfere with the action of some prescription drugs, as well as a few nonprescription drugs.

For some medications, prescription AND non-prescription, “the juice increases the absorption of the drug into the bloodstream”. “When there is a higher concentration of a drug, you tend to have more adverse events.” This caution does not apply to injectable medications.

More about this: Mayo Clinic, Wikipedia.