Actually, 20/10 refers to 20mg of Dex, 10mg of Quinidine. The FDA didn't approve the 30/10. Both of them have the same amount of quinidine in them, so the risk for QT prolongation is the same.
The FDA didn't approve the 30/10, I am guessing, because of what I would assume is an increased risk of dizziness, as I imagine Dex makes people more unsteady. Of course I don't have the numbers offhand to look at them again.
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