Occam I checked the manufacturers package insert and it mentions nothing about an interaction with Grapefruit juice but the enzyme I was referring to above is known as CYP450. It is present in the liver and the lining of the intestine and it is known to be inhibited by some of the components of Grapefruit juice so I think its reasonable to assume that Grapefruit juice might result in a decrease in the activation of Clopidogrel.
Unfortunately I don’t know what the half life is of the inhibitory effect is so advising on a proper separation between grape fruit consumption and medication ingestion is difficult. The problem is this. There are several different compounds in grapefruit juice that effect the P450 family of enzymes although the most studied one is Bergamottin. Additionally not all of the enzymes in the P450 family are affected the same way by a given grapefruit compound. Finally each individual can vary markedly in how they are affected possibly due to minor differences in the structure of the enzymes they carry or perhaps related to how quickly that person produces new enzyme to replace the inhibited ones. In one study the change in the serum level of a particular blood pressure medicine changed by anywhere from -33% to 125% in different individuals after they consumed grapefruit.
If you really need to be on Clopidogrel and you want to be sure you are getting the proper dose you may need to give up grapefruit altogether or alternatively ask you physician if you need an anti platelet drug at all and if so is there an alternative you could use.