Mental disorders account for about a third of sick days, roughly equal to those caused by back and neck pain, according to the most comprehensive report yet on the effect of illness on disability.
The survey tallied chronic conditions but not transient illnesses like the flu.
Adult Americans with depression, anxiety or other psychological disorders annually miss 1.3 billion days of work, school or other daily activity, according to the report today in the Archives of General Psychiatry. Back and neck pain cause sufferers to miss 1.2 billion days, the report said.
Mental disorders had a bigger effect than expected, researchers said, yet they are often left unrecognized and untreated.
"If we treated the mental disorders, we could wipe out a lot of the impairment," said senior author Ronald C. Kessler, a professor of healthcare policy at Harvard Medical School.
Lead author Kathleen Merikangas, an epidemiologist with the National Institute of Mental Health, said more money should be spent on studying and treating musculoskeletal conditions and psychiatric disorders such as depression.
"It is ironic we spend the least on musculoskeletal disorders and depression when they have the most impact on people's lives and disability," she said.
The study was based on the National Comorbidity Survey-Replication, a nationwide survey of 9,282 adults that is sponsored by the National Institute of Mental Health.
Participants reported the number of days they had been completely unable to work or carry out their normal activities within the previous month.
They also said which of 30 mental and physical conditions they had had during the last year.
Taken together, all chronic conditions -- including cancer, heart attack, ulcer and vision loss -- sideline adults for 3.7 billion days a year, researchers said.
More than half of American adults have chronic health conditions, researchers found.
Some work; others are too impaired to.
The study found that those with chronic conditions took an average of 32 sick days a year.
Among mental disorders, depression accounted for the most sick days, 387 million. Other reasons included social phobia, 214 million days; post-traumatic stress disorder, 113 million; generalized anxiety disorder, 110 million; bipolar disorder, 103 million; panic disorders, 101 million; substance abuse, 93 million; agoraphobia, 37 million; and separation anxiety disorder, 20 million.
Besides back and neck pain, other physical conditions that led to missed days included arthritis at 375 million days and stroke at 221 million days.
Heart attack victims missed 204 million days and cancer patients missed 71.5 million.
The relatively low figures for cancer and heart attacks can be attributed to the fact that the diseases tend to strike older adults, Kessler said.
And cancer often does not affect the ability of people to function from day-to-day as much as back pain or depression.