Full-body scans expose people to similar levels of radiation as atomic bombs used in Hiroshima, say experts.
Radiation dose from a single scan was similar to Hiroshima doses
CT or computed tomography scans use ionising radiation to take pictures of the inside of the body and can detect things like cancer.
But they can also cause cancer, US scientists at Columbia University say in the journal Radiation.
They warned healthy people not to seek full-body CT scans as part of their health check-ups.
But they said in people with signs or symptoms of disease, the benefits of repeated scanning far outweighed any radiation exposure risks.
In the US, and to a lesser extent in the UK, a growing number of healthy people with no signs or symptoms of disease are undergoing repeated CT scans, in some cases annually, as part of medical MOTs.
Dr David Brenner and colleagues set out to estimate the risk repeated CT scans might pose using data on those exposed to radiation after the 1945 atomic bomb blasts at Hiroshima and Nagasaki.
The estimated radiation dose to the lung or stomach from a single, full-body CT scan was similar to that received by some of the survivors of atomic bombs who had been exposed to the lowest levels of radiation during these disasters.
These survivors are known to be at increased risk of cancer as a result of their radiation exposure, suggesting a similar risk is posed by CT scans, say the researchers.
"Our research provides definitive evidence that radiation risk is associated with full-body CT," said Dr Brenner.
Risks justified in some
They estimate if a 45-year-old had one scan, their lifetime risk would be one in 1,200.
In other words, if 1,200 people of this age had a single scan only one would go on to develop cancer as a result.
If a 45-year-old had a scan every year for 30 years, however, their risk would be about one in 50.
Dr Brenner said the risks would be justified if the scans were carried out in a person with symptoms suggesting cancer because of the benefits of identifying the tumour early and treating it.
But when used as a health check in someone with no reason to believe they had cancer, the risks of repeated scans were too high and outweighed any benefits.
Dr Paul Dubbins from the Royal College of Radiologists said the findings were valid.
He said there were a few private institutions in the UK, mainly in and around London, providing full-body CT scans to healthy people as a screening tool.
But he thought it was extremely unlikely people in the UK were having numerous scans unless they had symptoms or signs of disease that would need CT monitoring.
"If you had a cancer you might have three or four within the early stages of your treatment and then an annual scan thereafter," he said.
He said in that instance, the risks were acceptable.
"The risk of the radiation dose is much the lesser evil than missing recurrence of the tumour," he said.
Dr Dubbins estimates around 400 full-body CT scans are performed in the UK every day.
Recent research by Oxford University and Cancer Research UK estimates about 0.6% of total cancer risk may be due to exposure to X-rays in hospitals.