Stomach surgery to treat obesity is much riskier than had been thought, with patients facing a far greater chance of being hospitalized and dying following the increasingly popular operations, according to two large new studies.

One analysis of more than 60,000 California patients found they were twice as likely to require hospitalization after the operations than before, while the second study of federal data from more than 16,000 patients nationwide found the chance of dying after being released from the hospital was significantly higher than earlier studies indicated.

The studies are the largest to examine the risk of hospitalization and death after the surgery, which helps obese people lose weight by reducing the size of their stomachs.

"This is a reality check for people thinking of having obesity surgery," said David R. Flum of the University of Washington at Seattle, who led the nationwide study. "Patients really need to go into this with their eyes open. This can be a high-risk operation."