Shares of UnitedHealth Group Inc. took a historic plunge Thursday after the health insurer slashed its full-year outlook and missed quarterly earnings expectations for the first time in years, citing increased care activity in its Medicare Advantage business.

The worst plunge for UnitedHealth Group’s stock in more than a quarter century is weighing on Wall Street, even as most stocks ticked higher. The S&P 500 was 0.1% lower in its final day of trading for a holiday-shortened week.

UnitedHealth shares tumbled early Thursday after the company cut its 2025 forecast in the wake of a surprising rise in care use from its Medicare Advantage customers, leading to a worse-than-expected first quarter.

UnitedHealth said it was blindsided by rising medical costs as the first quarter closed, upending a forecast it had affirmed just three months ago.