What is it about grapefruit that most people on heart medication cannot eat them?

Grapefruit contains high levels of a compound called furanocoumarins, which can interfere with the way certain heart medications are metabolized in the body. Furanocoumarins give grapefruits their bitter or sour taste and also act as natural insecticides.

When you consume grapefruit, these furanocoumarins interact with enzymes in your intestine to block an important enzyme called CYP3A4. This enzyme is responsible for breaking down certain drugs that people take for their hearts, including statins and calcium channel blockers. In essence, when you eat grapefruit while taking heart medication, more of that medication ends up circulating in your bloodstream than normal because it isn't being broken down properly by CYP3A4.

This can increase the drug's activity level beyond what was intended and may cause serious side effects such as an abnormally slow or irregular heartbeat, a sharp drop in blood pressure (hypotension), dizziness, fainting spells (syncope) or kidney failure. In rare cases it can even lead to death if left untreated.
For this reason doctors generally recommend avoiding all forms of grapefruit products when taking any type of cardiovascular medication such as angiotensin-converting enzyme inhibitors (ACEs), beta-blockers or statin drugs like atorvastatin (Lipitor).