The chief executive of Pfizer, Jeffrey Kindler, has announced his surprise departure from the world's largest drugmaker to "recharge my batteries."
He is being replaced by Ian Read, 57, the global head of pharmaceuticals.
In his resignation statement, Mr Kindler, 55, acknowledged the personal toll of running a major organisation.
He said that the "24/7 nature of my responsibilities" had been "extremely demanding", especially after last year's $67bn (£42bn) takeover of Wyeth.
Mr Kindler, chief executive for five years, has overseen a 26% fall in Pfizer's share price, far stripping the 10% fall in the Arca Pharmaceutical index of major drugmakers.
He was formerly the company's legal counsel, and was seen as a surprise choice because of his relative lack of pharmaceuticals experience.
Analysts said Pfizer needed an injection of new blood, especially as it approaches a major challenge - the expiry in the US of its bestselling Lipitor cholesterol drug.
"The departure is sudden, but I doubt there was one event per se" that led to his resignation, said Tim Anderson, analyst at Sanford Bernstein.