- Computed tomography (CT) is readily available in developed countries. As one of the side effects includes an increased risk of cancer, interventions that may encourage more judicious use of CT are important. Behavioural economics theory includes the use of nudges that aim to help more informed decisions to be made, although these have been rarely used in hospitals to date. We aimed to evaluate the impact of a simple educational message appended to the CT report on subsequent numbers of CT completed using a controlled interrupted time series design based in two teaching hospitals in the UK. The intervention was the addition of a non-directional educational message on the risk of ionising radiation to all CT reports. There was a statistically significant reduction in the number of CT requested in the intervention hospital compared to the control hospital (-4.6%, 95% confidence intervals -7.4 to -1.7, p=0.002) in the 12 months after the intervention was implemented. We conclude that a simple, non-directional nudge intervention has the capacity to modify clinician use of CT. This approach is cheap, and has potential in helping support doctors make informed decisions.