My understanding is that Simvastatin (Zocor) has greater metabolism through CYP3A4 then Atorastatin (Lipitor). So, Grapefruit juice will have a greater effect on someone who takes Simvastatin then who takes Atorastatin.
With the Statin drugs (in particular, since the OP never did mention what drug was in question) grapefruit inhibits the metabolism of 'statins, so they build up in the blood. This causes a higher concentration of the drug then was prescribed.
This is a concentration depend process, so it matters what level of Grapefruit Juice there is, and also what the strength of the 'statin is.
In the case of this OP, since the Grapefruit juice is listed so low in the ingredient list, I would think it would be perfectly ok to continue drinking Sierra Mist.