As a consequence, the prescribed dosage of a medication which is affected by the enzyme must be increased substantially to assure that the needed amount is absorbed. While I do not know for certain, it is plausible that the prescribed dosage was determined in clinical trials which would account for differences among people and still provide appropriate safeguards. Some reports show increases by 300 to 500 percent, some far more. For clarification, that is percent, not times in other words, a 300 percent increase is the same as three times as much, not 300 times as much. Some medications now state that users should avoid all grapefruit consumption.
Grapefruit also increases absorption of caffeine, one form of estrogen and an anticonvulsant.
Viagra is also metabolized by the CYP3A4 enzyme, so it may also be impacted. One would hope that, ultimately, pharmaceutical companies could safely recommend lower dosages for grapefruit lovers, especially in light of the tremendous benefits now being attributed to grapefruit, but I wouldn't count on it. In any case, if you are concerned about possible interactions with grapefruit, always check with your physician. I opened only those dated since late 2000, and quit well shy of half the entries. As a final irony, grapefruit, among other fruits and vegetables, is so critical in the prevention of many of the health problems that afflict us today yet can cause complications in the medicinal treatment of those problems.