Working for a company that expands rapidly increases an employee's risk of long-term sickness, say researchers.
The Lancet study by experts from the Swedish National Institute for Psychosocial Medicine also found hospital admission rates increased.
The team found women working in the public sector were worst affected.
Experts suggest while expansion may be thought to be beneficial, growing firms may have too few qualified staff and feel generally unstable.
Previous research looked at the negative health effects of downsizing, but the Swedish team believe theirs is the first to look at the opposite phenomenon. They studied the employment records of 24,000 healthy workers aged under 65 in 1999.
They looked at long-term sick leave (of over 90 days) and hospital admission rates.
Significant company expansion, classed as an increase in the number of employees of 18% or more in a year, was found to be linked to an increased risk of long-term sickness absence and hospital admission.
Women in the public sector who were exposed to large expansion every year over the six years of the study had a two to three times higher risk of long-term sickness absence than those who were never exposed to large expansion.
Researchers suggest this could be because public sector jobs such as healthcare and education, commonly dominated by women, have been linked to increased demands and hazards.
Being exposed to moderate expansion (between 8 and 18% increase in staff in a year) resulted in a decreased risk of hospital admission. 'Flexibility'
Lead researcher Dr Hugo Westerlund said: "That the effects of rapid expansion on health are negative is somewhat surprising, since expansion is usually thought to imply high job security and a positive, successful context, which should promote good health.
"A positive effect of exposure to moderate expansion can indeed be seen in relation to hospital admissions.
"However, large and rapid expansion could lead to problems such as difficulty in recruiting enough qualified personnel and a generally unstable and less than ideal organisational structure."
He added: "High workload, plus insufficient practical and social support could be pathways between these organisational factors and health."
In an editorial, the Lancet commented that modern employees are expected to be flexible, open to change and accept that a long-term relationship with an employer can no longer be expected.
It added: "What's the solution? Value people over profits? Recognise human capital as paramount? These answers are obvious.
"But given the high-handed, soulless treatment doled out today by many employers, it's a start.
"If appeals to integrity and character fail, there's always the bottom line: depressed and anxious personnel are unlikely to be productive, and absence from work costs employers and society money.
"The re-engineering of work ought to perpetuate fulfilment and productivity in employees, not illness and disability."
TUC General Secretary Brendan Barber said: "Every year stress shows up in surveys as a key workplace issue and change is one of the top five causes of stress.
"People working for firms undergoing massive change are understandably going to feel nervous about their futures.
"This is where employers should be managing change as well providing advice and support on relocation, job alternatives, counselling and tips on how to avoid and tackle stress." BBC News