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The findings raise questions about whether people with serious mental illness receive appropriate cancer screenings and preventive care to help them avoid cancer risk factors such as smoking, the researchers said.
"The increased risk is definitely there, but we're not entirely sure why," study leader Dr. Gail Daumit, an associate professor of medicine and psychiatry at the Johns Hopkins University School of Medicine, said in a Hopkins news release. "Are these people getting screened? Are they being treated? Something's going on."
The researchers analyzed data from more than 3,300 Maryland Medicaid beneficiaries with schizophrenia and bipolar disorder to determine how many of them were diagnosed with cancer between 1994 and 2004.
Compared to people in the general population, schizophrenia patients were more than 4.5 times more likely to develop lung cancer, 3.5 times more likely to develop colorectal cancer, and nearly three times more likely to develop breast cancer, the researchers found.
Patients with bipolar disorder had similarly increased risk for the three types of cancer, according to the study published in the July issue of the journal Psychiatric Services.
People with serious mental illness are more likely to smoke, which could explain their elevated risk for lung cancer, Daumit said.
She also noted that women with schizophrenia and bipolar disorder are less likely to have children and that childbearing is believed to reduce breast cancer risk. In addition, some drugs used to treat mental illness can increase levels of the hormone prolactin, a factor that has been linked to breast cancer.
The increased risk of colorectal cancer could be due to lifestyle issues such as smoking, lack of exercise and a diet lacking fruits and vegetables, Daumit said.
While the study uncovered a link between mental illness and cancer risk, it did not prove that one causes the other.
In a separate study released last month, Daumit found that people with serious mental illness were nearly twice as likely to require emergency or inpatient department treatment for an injury than people in the general population, and were about 4.5 times more likely to die from their injuries.
The study was published online in the journal Injury Prevention.
About 5 percent of Americans have a serious mental illness and these people are known to have a two to three times increased risk of dying prematurely, Daumit noted.
-- Robert Preidt
Copyright © 2012 HealthDay. All rights reserved.
SOURCE: Johns Hopkins Medicine, news release, July 18, 2012