At least 116,000 Iraqi civilians and more than 4,800 coalition troops died in Iraq between the outbreak of war in 2003 and the US withdrawal in 2011, researchers estimate.
Its involvement in Iraq has so far cost the United States $810 billion (625 billion euros) and could eventually reach $3 trillion, they added.
The estimates come from two US professors of public health, reporting on Friday in the British peer-reviewed journal The Lancet.
They base the figures on published studies in journals and on reports by government agencies, international organisations and the news media.
"We conclude that at least 116,903 Iraqi non-combatants and more than 4,800 coalition military personnel died over the eight-year course" of the war from 2003 to 2011, they said.
"Many Iraqi civilians were injured or became ill because of damage to the health-supporting infrastructure of the country, and about five million were displaced.
"More than 31,000 US military personnel were injured and a substantial percentage of those deployed suffered post-traumatic stress disorder, traumatic brain injury, and other neuropsychological disorders and their concomitant psychosocial problems."
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Citing figures from the website costofwar.com, which looks at funding allocated by Congress, the study said that as of January 15 this year, the Iraq War had cost the United States about $810 billion, "not including interest on debt."
"The ultimate cost of the war to the USA could be $3 trillion," it said.
"Clearly, this money could have been spent instead on domestic and global programmes to improve health. The diversion of human resources was also substantial, in Iraq, the USA, and other coalition countries."
The paper is authored by Barry Levy of Tufts University School of Medicine in Boston and Victor Sidel of the Albert Einstein College of Medicine in New York.
It appears in a package of investigations into the health consequences of the Iraq War, published by The Lancet to mark the 10th anniversary of the start of the conflict.
In 2006, estimates by researchers at Johns Hopkins University in Baltimore, Maryland, also published in The Lancet, said 655,000 people had died in the first 40 months of the war. That figure was widely contested.
In 2008, a study by the Iraqi government and World Health Organisation (WHO), published in The New England Journal of Medicine, said between 104,000 and 223,000 Iraqis had died violent deaths between March 2003 and June 2006.
Those figures were based on home visits to around 1,000 neighbourhoods across the country.