Stephen Hemsley ran UnitedHealth Group for more than a decade. It's a different company after four years with Andrew Witty in charge.

Even UnitedHealth will struggle to overcome parasitic medical costs that Warren Buffett once called a tapeworm eating away at U.S. economic competitiveness. The $300 billion healthcare conglomerate reinstalled Chairman Stephen Hemsley as CEO and yanked its financial guidance.

Former CEO and current chairman Stephen Hemsley is stepping back into the top job following the abrupt resignation of Andrew Witty, UnitedHealth said.

UnitedHealth Group CEO Andrew Witty abruptly stepped down from the health insurance and care conglomerate on Tuesday, citing personal reasons, capping the end of a tumultuous four-year tenure that included the killing of the company’s top insurance executive .

UnitedHealth CEO Andrew Witty is stepping down for personal reasons and the nation’s largest health insurer suspended its full-year financial outlook due to higher-than-expected medical costs. Hemsley was UnitedHealth Group CEO from 2006 to 2017. He will remain chairman of the company’s board. Witty will serve as a senior adviser to Hemsley.

UnitedHealth Group surprises investors with what its CEO said was an "unusual and unacceptable" quarterly earnings miss, and lowers its outlook for the full year due to higher-than-expected medical costs, sparking a more than 20% selloff in shares that reverberated across the sector.