For certain people, many kinds of medication become much more powerful if taken with grapefruit or grapefruit juice. The increase in potency could be especially dangerous with drugs that affect the blood pressure and heart rate, lower the blood levels of sugar or calcium, and with sleeping pills and other drugs that become toxic at high dosages (e.g., some drugs taken by mouth for fungal infections). In this way, some people have accidentally overdosed.
This increase in potency is not just imaginary -- researchers have found that something in grapefruit inhibits intestinal enzymes that would otherwise break down certain drugs. So, when purchasing any medication for the first time, be sure to read the pharmacy's printout (especially the precautions and warnings section) to find out whether grapefruit would affect them. Alternatively, to be on the safe side when taking medication, you could temporarily do without grapefruit or grapefruit juice.
Editor's note: Debbi Hernandez, pharmacist who spoke at our Aug 1999 meeting, noted that grapefruit doesn't always intensify effects of medications; it sometimes reduces them as well.