Researchers tracked the patients three months after their COVID-19 diagnosis and compared their mental health diagnoses to those of thousands of other patients with differing conditions, such as the flu and broken bones.
The findings revealed that 18% of the sampled COVID-19 patients were diagnosed with a psychiatric illness, while only 13% of those with the flu and 12.7% of those with a fracture were diagnosed with mental illness.
The researchers also found that 5.8% of the COVID-19 patients were diagnosed with a mental disorder for the first time, compared to 2.8% and 2.5% of patients with the flu and a fracture, respectively.
The most common psychiatric illness diagnosis among COVID-19 patients was anxiety, including adjustment disorder, generalized anxiety disorder and post-traumatic stress disorder. Mood disorders, such as depression, and insomnia were the next most-common diagnoses.
In addition, researchers identified significantly higher risks of dementia in COVID-19 patients. Dementia is associated with loss of memory, language, problem-solving and other thinking abilities.
"We urgently need research to investigate the causes and identify new treatments," said University of Oxford psychiatry professor Paul Harrison, the lead study author, the BBC reported.
The study also found that people with a pre-existing mental illness were 65% more likely to be diagnosed with COVID-19 than those without one.
Dr. Michael Bloomfield at University College London, who was not involved in the study, told the BBC that the link between COVID-19 and mental illness was probably due to "a combination of the psychological stressors associated with this particular pandemic and the physical effects of the illness.”
However, Jo Daniels at the University of Bath said that additional research must be completed before drawing any concrete conclusions.
"We should be aware that poorer psychological outcomes are common in those who experience physical health problems of any nature," she told the BBC.
"Being acutely or chronically unwell is simply a difficult experience,” Daniels added.