Actually of all the topics Krugman posts on, he is probably the least clueless on healthcare, but his case would be better supported if he wouldn't make exaggerated comments like this.
...[D]rug companies in particular spend more marketing their products to doctors than they do developing those products ... They wouldn't do that if doctors were immune to persuasion.
Now drug companies do spend a considerable amount on marketing to doctors, by one report an estimated $5.7 billion a year, promoting your products is a cost of doing business after all, but but by no means is this more than they spend on development. Just one company, Pfizer, spent more than that, $7.7 billion on R & D last year. You would think an award winning economist would know how to read a corporate earnings report.
But even this overstates it, about half of the money spent on marketing drugs to doctors is giving them free samples to give to patients! So I guess if the government gives drugs to patients for free that is good, but if the companies do that, it is bad.