NEW YORK -- The head of the world's biggest drugmaker, Pfizer Inc., has stepped down unexpectedly after less than five years at the helm.

Jeffrey B. Kindler, a lawyer who joined Pfizer in 2002, says in a surprise announcement that he's retiring after what he calls 4 1/2 "extremely demanding" years to recharge his batteries.

Kindler, 55, says he plans to spend more time with family for a while and prepare for new challenges.

His successor, Ian Read, takes over immediately. Read, 57, has run Pfizer's worldwide pharmaceutical operations since 2006. That business brings in most of Pfizer's revenue.

It sells everything from blockbuster cholesterol drug Lipitor and impotence pill Viagra to cancer drugs and off-patent medicines that still sell well in some countries.