Investors and analysts have been taken by surprise at the announcement this morning that Jeffrey Kindler is stepping down as Pfizer's chief executive and chairman.

Mr Kindler, who has been CEO for four-and-a-half years, leaves after overseeing the drug giant's $67 billion acquisition of Wyeth. However, at 55, it seems that the job has taken its toll.

He said that "the combination of meeting the requirements of our many stakeholders around the world and the 24/7 nature of my responsibilities, has made this period extremely demanding on me personally". Mr Kindler went on to say that "I am excited at the opportunity to recharge my batteries, spend some rare time with my family and prepare for the next challenge in my career".

He is being succeeded as CEO by Mr Read, 57, who is currently head of Pfizer's global biopharmaceutical operations. The latter will also take on the post of president and the company said the board will elect a non-executive chairman from its current membership within the next two weeks.

The board's lead independent director, Constance Horner, said that "today's business leaders need to understand global markets, drive change and innovation, and move quickly to adapt to competitive pressures. Ian's track record throughout his career has demonstrated these exact strengths".

That track record will be tested to the full as Pfizer prepares for life after Lipitor (atorvastatin), the cholesterol blockbuster which will lose patent protection in the USA next year. The manner of Mr Kindler's departure differs greatly to the way rival Merck & Co revealed last week that its president Kenneth Frazier would succeed Richard Clark as CEO, a move that had long been predicted.

Pfizer investors are expected to be less than dismayed by the news, given that since Mr Kindler took over, the company's shares have lost over a quarter of their value and it has suffered several clinical failures.