Okay, but just to be clear in case you did not do even a quick Google search, it's specifically grapefruit that can hurt/kill you. Not limes or lemons or most oranges, grapefruit. Grapefruit messes with metabolizing enzymes responsible for fully breaking down and flushing drugs, which means those drugs can stick around in you for a lot longer and in higher concentrations than they should, causing more severe side effects and/or possibly even overdoses. Here's a random excerpt from a random article that at least gets the idea across:
With most drugs that are affected by grapefruit juice, “the juice lets more of the drug enter the blood,” says Shiew Mei Huang, Ph.D., of the FDA. “When there is too much drug in the blood, you may have more side effects.” For example, if you drink a lot of grapefruit juice while taking certain statin drugs to lower cholesterol, too much of the drug may stay in your body, increasing your risk for liver and muscle damage that can lead to kidney failure. Many drugs are broken down (metabolized) with the help of a vital enzyme called CYP3A4 in the small intestine. Grapefruit juice can block the action of intestinal CYP3A4, so instead of being metabolized, more of the drug enters the blood and stays in the body longer. The result: too much drug in your body. The amount of the CYP3A4 enzyme in the intestine varies from person to person. Some people have a lot of this enzyme and others just a little. So grapefruit juice may affect people differently even when they take the same drug.
Although scientists have known for several decades that grapefruit juice can cause too much of certain drugs in the body, more recent studies have found that the juice has the opposite effect on a few other drugs. “Grapefruit juice can cause less fexofenadine to enter the blood,” decreasing how well the drug works, Huang says. Fexofenadine (brand name Allegra) is available as both prescription and OTC to relieve symptoms of seasonal allergies. Fexofenadine may also not work as well if taken with orange or apple juice, so the drug label says, “Do not take with fruit juices.” Why this opposite effect? Instead of changing metabolism, grapefruit juice can affect proteins in the body known as drug transporters, some of which help move a drug into our cells for absorption. As a result, less of the drug enters the blood and the drug may not work as well, Huang says.
Hope you make it through the medical issue you're dealing with. If it's chemotherapy (and I don't know that it is and nobody here needs to know if that's what it is...I've just had family and friends who have gone through it and have repeatedly said how awful it is, and how it makes it so they simply cannot eat), there's a strong possibility you do not want to be having grapefruit with it - everywhere I see seems to say "DO NOT MIX CHEMOTHERAPY WITH GRAPEFRUIT". If your doctor has okay-ed grapefruit, then okay.