UnitedHealth Group shared its economic outlook on Thursday, with its CEO saying it was an “unusual and unacceptable” quarterly earnings miss.

Shares have dropped over 20%, the largest seen by the Minnesota-based insurance giant in over 26 years. UnitedHealth Group also said it had its first quarterly miss since 2008.

According to the company’s quarterly earnings report, the unprecedented downturn is due to rising medical costs, specifically Medicare expenses concentrated in physician and outpatient services.

UnitedHealth also said the company’s performance was affected by patients seeking care at a higher rate than expected.

“UnitedHealth Group grew to serve more people more comprehensively but did not perform up to our expectations, and we are aggressively addressing those challenges to position us well for the years ahead, and return to our long-term earnings growth rate target of 13 to 16%,” said CEO Andrew Witty in a statement.

UnitedHealth reported adjusted first quarter earnings of $7.20 per share, missing its expected $7.29 per-share estimate. Shares were also down around 23% by Thursday afternoon on heavy trading.

Its revised annual adjusted earnings outlook will be $26 to $26.50 a share this year, down from UnitedHealth’s original range of $29.50 to $30 per share.

Additionally, the healthcare giant cited “unanticipated changes” in its Optum Health care delivery business that has affected reimbursements. Optum Health had been UnitedHealth’s fastest-growing source of profit up until this recent earnings report.

Optum operates home care business, clinics and surgery centers.

Despite the quarterly downfall, UnitedHealth Group still reported a revenue increase of nearly $10 billion, whereas Optum reported nearly $2 billion in losses.