The problem with grapefruit, as well as Seville oranges, pomelos and limes, is that some substances found in the juice inhibit an enzyme found in the intestines and liver that aids in metabolizing about 35 often prescribed drugs. The result of this is that you might end up with a higher level of the active ingredient in your bloodstream. This can lead to adverse effects. Some can be serious and might result in acute kidney failure, heart problems, trouble breathing and gastrointestinal bleeding.
This interaction occurs quickly and can last 24 hours or longer. The effects vary from person to person, but older people are at higher risk. The danger is also greater for drugs that have little forgiveness if the dose is off by even the slightest amount.
The bottom line if you take a prescription medication, is to read all of the instructions carefully and if you have any doubt at all check with your health care professional.