UK patients are being put at risk because of nursing staff cuts and excessive workloads, the Royal College of Nursing (RCN) said.
The union said there is "unprecedented" stress on nurses which is leading to many of them suffering from ill health.
A poll conducted by the RCN on 2000 nurses working across the NHS and private sector found that more than half have been made unwell by stress in the past year.
And four in five of said they have battled through feeling unwell and still gone into work.
The RCN said that many nurses reported that sickness absence policies at their place of work were so punitive that they had no alternative but to attend work despite feeling unwell.
One of the nurses questioned told the RCN: "I am currently off work following breast cancer, a senior manager called three weeks after my surgery and asked if I was coming back as people with cancer often don't return and they wanted to fill my post."
Dr Peter Carter, general secretary of the RCN, said: "Our whole care system is currently facing the huge challenge of delivering care at a time of increased demand and scant resource. There is much that managers and employers can do, and should be doing, to support staff and keep patients safe."
A Scottish Government spokesman said: "The nursing workforce planning tools, which became mandatory from April 1, will help ensure that hospitals have the right number of people and right skills mix to deal with demand."