Metoprolol is inhibited by the CYP 2D6 enzyme, which is inhibited by many other medications (among others most SSRIs, except Zoloft)
The danger with grapefruit
and especially grapefruit juice
(as you can easily drink juice
equal to 6-7 grapefruits) is the combination with antiarrhythmics causing a prolonged QT interval, which have very narrow therapeutical windows (low difference between dosage and overdosage) or other substances prolonging the QT interval (certain antibiotics, etc.