I don't even mean to suggest lying. I mean something simple like "I think this drug might help me concentrate."

A formal diagnosis of ADD or narcolepsy is carte blanche for amphetamine prescription. Because it is highly scheduled and, moreover, has a big black market, doctors guard this diagnosis carefully. Whereas, modafinil is lightly scheduled and doesn't have a black market (not driven by prescriptions), so they are less nervous about giving it out in ADD-ish situations.

But doctors very much do not like it when a new patient comes in asking for a specific drug.