UnitedHealth’s cut in full-year profit guidance sparked a 20% selloff in shares|Chad Davis|CC BY 2.0

Shares of UnitedHealth fell Thursday—its biggest single-day drop since August 1998—when it cut its forecast for 2025 due to higher-than-expected medical costs. The issue has affected the entire industry.

The cut in full-year profit guidance sparked a 20% selloff in shares. It is the company’s first earnings miss since 2008.

The company said the recent changes to Medicare plans have more than doubled, which it had not anticipated, and the move has reduced insurers’ revenue.