Mental health problems, such as stress, depression and anxiety, are the most common reason for NHS staff in England to get sick for days, according to NHS Digital statistics.
Employees received a total of 17.7 million days of sick leave between December 2017 and November 2018.
Of these, almost a quarter or 4.2 million were taken because of anxiety, anxiety, depression or other mental illnesses.
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This was more than the next two most common reasons combined, including musculoskeletal disorders and the common cold.
The Sheffield Health and Social Care Foundation had the highest percentage of lost days in depression and other psychiatric conditions, with 38% attributable to these conditions.
The findings prompted the UK's largest health workers' union, Unison, to call on the government to invest more money in NHS services.
The deputy head of the health union, Helga Peele, said addressing unacceptable work pressure, bullying and bullying and violence by patients were just some of the reasons so many NHS workers were dealing with mental health.
"The shortage of staff means that NHS staff are usually called upon to do more with fewer resources as they desperately try to keep the service afloat," he added. "The government must urgently invest in the NHS to reduce staff shortages and reduce burnout and workers suffering from anxiety, depression and stress need to have quick access to mental health support services."
More than half of all workers in all industries say they are affected by poor mental health in their workplace, according to a recent study by the mental health charity Mind.
Emma Mamo, head of workplace wellness, said it was important for employers to support struggling workers and address work-related mental health causes, especially in the NHS.
"We know that there can be special barriers for healthcare staff when they reveal a mental health problem to their employer, such as fears that they may be considered inappropriate for exercise," he said. "Those of us with mental health issues can and do make a valuable contribution to the workplace, meaning some of us may need extra support from time to time.
"Healthcare staff can make a real difference to the experiences of people accessing NHS services."
Responding to the findings, a NHS England spokeswoman said she was committed to taking care of her staff and offered "support, good occupational health, flexible work and a range of other measures".
They added: "Staff are the NHS incest and we are already offering the most comprehensive national mental health support offer to doctors of any health system in the world and are committed to doing the same with other staff groups."