It is not that grapefruit :interferes".
Grapefruit juice contains substances that prolong the half-life of a medication in the blood.
The difference between a dangerous dose of a medication and a therapeutic dose may involve a very small additional concentration.
Thus, for some medications there is a caution on the bottles prohibiting ingestion of grapefruit juice.
Yes, it interferes with a lot of meds.
Yes. read this for information - http://www.mayoclinic.com/health/food-and-nutrition/AN00413