What I'm going to say is likely very inflammatory, but I believe it to be an accurate statement:
Usually the child acts in that manner because he/she was trained that way. Medicines and labeling are overused.
It may be worth taking another looking at your parenting techniques. Do you ever look at other parents with their children and think, "Why do they let their kids do that?" Many other parents are looking at you and likely thinking the same thing. When I wanted to correct and improve my parenting abilities, I really liked the book "One, Two, Three, Magic" This one allowed me to be far more consistent. My previous lack of consistency caused a lot of problems. Once things were more relaxed then I started using many of the techniques in "Parenting With Love and Logic". This allowed me to teach my daughter to help make age appropriate decisions and realized the consequences of her actions. It also showed me how to better use natural consequences which are far more effective than most of the consequences, both positive and negative, that I could give her.
_________________
Things aren't always the way they seem-- At least that's the way it seems