I know what off-label marketing is perfectly well, but I'm not sure that you do. A drug might very well have benefits that are not approved by the FDA, and an attempt to "sell" a doctor on those benefits is what's called off-label marketing.
For instance, one of the off-label uses for this drug is alleviating nausea. So in addition to its approved use--anti-psychotic--it happens to also alleviate nausea. Only the FDA hasn't approved it as an anti-nausea medication. But whether or not the FDA has approved it as anti-nausea medication doesn't change the nature of the drug, and that it does, in fact, suppress nausea.
So what goes on with off-label marketing is that the prescription drug salesman is out at doctors' offices and telling them all about how this is a great anti-psychotic drug or whatever. The doctor says fine, fine, I'll prescribe it to my patients that need that. Then the salesman says something like, "hey, by the way, this drug also happens to alleviate nausea. Not saying you should prescribe it for that, but I just thought you might like to know."
Then a patient comes in and is having trouble with constant nausea and the doctor prescribes the drug. Nausea ends, because that's what the drug does--alleviates nausea. Patient is happy--nausea is cured. Doctor is happy, because his patient isn't nauseated any longer. Drug company is happy because it sold more drugs. FDA is sad because off-label marketing threatens its role as the great drug Oracle.