Apart from outright corruption there are many important reasons for regulation of the pharmaceutical sector. The links below are to a few stories that indicate the need for regulation.
Pharmaceutical companies make huge profits and as a sector it is matched only by the banking sector for the largest profits. Many of the worlds best known pharma companies spend more on marketing then they do on R&D: http://www.bbc.com/news/business-28212223
The US Justice Department is so concerned about the marketing prowess of the pharma companies that it released a 2001 recording of a sales executive meeting for GSK which you can view here and the first 30 seconds speaks for itself.
When the US government brought in the affordable care act they also launched a website that patients/public could search to see what perks their doctor has received. It can be found here: https://openpaymentsdata.cms.gov/ A report into the site found some suprising statistics: http://projects.propublica.org/docdollars/ The types of work doctors do for pharma compneis include partaking in clinical trials, giving speeches and directly workign for a company.
Here is a list of what the fees to doctors are paid for: http://projects.propublica.org/docdollars/companies