It's naive to think that advertising isn't playing a significant role in what gets prescribed. These corporations aren't going to spend $4.5 billion without a very good reason. To state the obvious...the reason for advertising is to sell a product.
To stir the pot a bit, how about all those pharma reps clogging up doctors offices. They're wining and dining and maybe 69ing every health care provider they can get to in order to get their product moving. And their higher paid coworkers are visiting every public hospital and private hospital or the people involved in approving and purchasing pharmaceuticals.
http://www.scribd.com/doc/17599937/Hospital-Selling