You are right Sunny grapefruit (and to a lesser degree other citrus fruit) is one of a small group of foods that can cause dangerous side effects if eaten whilst taking certain medications.
Grapefruit has an enzyme that causes the drug to be absorbed too quickly. Any prescription medication that causes this sort of reaction will have it clearly labelled on the bottle and your pharmacist should warn you of this before you begin the medication. The drugs that this affects are some blood pressure tablets, cardiac medication, some antidepressants and antipsychotics. It can also affect the absorption of estrogenics (and possibly some other hormones) and the ocp. You need to remember that when you drink grapefruit (or any other juice) you are getting a large dose of the fruit. It would probably be wise to check out the grapefruit/herb connection but to my knowledge there are no known adverse interactions - having said that I am wondering if I can remember dong quai shouldn't be taken with grapefruit.... I will look it up and get back to you.