Anyone taking medication for ADD can tell you about appetite suppressants, I lost about 20 pounds after I got on medication. The medication they prescribe for it is pretty much like speed and the medication they used to prescribe for losing weight, from the best I understand.
I don't know how the over-the-counter stuff works, but the doctors used to do blood tests to make sure my liver was okay for about a year after I started taking the medicine. So you may want to mention something to a doctor before taking anything too serious.