Grapefruit juice has many medicinal uses including acidify urine, provide vitamin C which can help prevent colds and flu-like symptoms, and has ingredients that may be beneficial in preventing cancer. However, the juice for all its benefits may have a deleterious interaction with many medications.
Research about the interaction of grapefruit juice with drugs suggests that compounds in grapefruit juice, called furanocoumarins (for example, bergamottin), may be responsible for the effects of grapefruit juice. Researchers believe that furanocoumarins block the enzymes in the intestines that normally break down many drugs. One glass of grapefruit juice could elicit the maximum blocking effect, and the effect may persist for longer than 24 hours. Since the effects can last for such a prolonged period of time, grapefruit juice does not have to be taken at the same time as the medication in order for the interaction to occur. Therefore, unlike similar interactions, where the interaction can be avoided by separating the administration of the two interacting agents by a couple of hours, administration of grapefruit juice with susceptible drugs should be separated by 24 or more hours to avoid the interaction. Since this is not practical for individuals who are taking a medication daily, they should not consume grapefruit juice when taking medications that are affected by grapefruit juice.
The grapefruit juice-drug interaction can lead to unpredictable and hazardous levels of certain important drugs. For example Viagra is of special interest to men. The clinical information is incomplete, but men who take Viagra should be aware that grapefruit juice might boost blood levels of the drug. That could be a good thing for some men with erectile dysfunction, but it could trigger headaches, flushing, or low blood pressure.
Bottom Line: Grapefruit juice can be of benefit to many people who drink it on a regular basis. However, the juice can impair the metabolism of many medications thus placing juice drinkers at risk for developing higher than normal blood levels of their medication. I suggest if you are a frequent grapefruit juice drinker, that you check with your doctor.