Why can't you eat or drink grapefruit when taking antirejection medication after a liver transplant?

Lowers Drug Levels. Grapefruit causes special enzymes in the liver to become more active. These enzymes can cause accelerated breakdown of some of the medicines used to prevent rejection. If the drug levels drop unexpectedly, you could have a rejection episode and risk losing your transplant.
Poor Absorption. Grapefruit juice may interfere with the absorption of certain anti-rejection medications. This can leave you vulnerable to rejection.