There are 44 prescription drugs on the market today that should never be combined with grapefruit. That's because the sour fruit (and some other, closely related, kinds of citrus) contain chemical compounds called furanocoumarins that prevent your body from metabolizing certain prescription drugs. Essentially, the grapefruit creates an artificial overdose where one tablet packs the power (and side effects) of 20. The CBC has a full list of the drugs, which includes cancer drugs, cholesterol-lowering drugs, and drugs to treat problems of the urinary tract. Wikipedia has more about why this interaction happens.