tamazaball
It has nothing do do with citrus, rather 6′,7′-Dihydroxybergamottin, a compound in grapefruit, which inhibits cytochrome P450 in the liver, which is a family of proteins with extensive metabolism for many medications, that is involved. Therefore, since the usual metabolism of these compounds is inhibited, bioavailability is increased (they stay longer in the system), which can reach toxic levels, similar to overdosing.