Rost left Wyeth for Pharmacia in June 2001 and took a role leading its endocrinology division, and said that he soon began to be concerned from a business perspective about sales of Genotropin, Pfizer's human growth hormone drug, which had plateaued; Pharmacia's decision to pour money into off-label marketing to adults was not paying off, due to the low doses that adults took. In the next year, Rost became aware that the strategy was not only unwise, but was probably illegal, and began raising objections internally to try to get the company to change course. In July 2002 merger discussions with Pfizer were announced, and Rost and other executives briefed the Pfizer acquisition team on the issues with Genotropin as the merger talks progressed. However, when news broke in January 2013 of Rost's lawsuits against Wyeth (see below), things turned sour; Rost reported getting an email from his future Pfizer boss saying "There will not be a fit with the marketing organization" for him. The merger with Pfizer was completed in April 2003. Rost was offered a severance package, as other Pharmacia executives were, but Pfizer and Rost failed to come to agreement, and Pfizer did not fire him, as they were aware of the suit he filed against them related to Genotropin marketing (see below). His staff of 60 was slowly reassigned and by the end of summer 2003, he was left with his administrative assistant, who was reassigned in the summer of 2004, and his office was moved to an out of the way location.