Depression is more damaging to everyday health than chronic diseases such as angina, arthritis, asthma and diabetes, researchers say.
And if people are ill with other conditions, depression makes them worse, the researchers found.
"We report the largest population-based worldwide study to our knowledge that explores the effect of depression in comparison with four other chronic diseases on health state," the researchers wrote in the Lancet medical journal.
Somnath Chatterji of the World Health Organisation, who led the study, said researchers calculated the impact of different conditions by asking people questions about their capacities to function in everyday situations- such as moving around, seeing things at a distance and remembering information.
The researchers assigned a number between 0 and 100 reflecting a person's relative health score.
"Our main findings show that depression impairs health state to a substantially greater degree than the other diseases," the researchers wrote.
The team used World Health Organisation data collected from 60 countries and more than 240,000 people to show on average between nine per cent and 23 per cent had depression in addition to one or more of four other chronic diseases - asthma, angina, arthritis and diabetes.
The most disabling combination was diabetes and depression, the researchers said.
"If you live for one year with diabetes and depression together you are living the equivalent of 60 per cent of full health," Chatterji said in a telephone interview.
The findings show the need to provide better treatment for depression because it has such a big impact on people with chronic illnesses, Chatterji added.
"What tends to happen is a health provider doesn't look for anything else but the chronic illness," he said.
"What we are saying is, these people will also be depressed and if you don't manage the depression you can't improve a person's health because depression is actually worsening it."
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