As has effectively already been indicated, it will up to the consultant who asked for the CT scan to determine whether there's any advantage to injecting you with anything, either beforehand or during the scan itself..
With the CT scan I had (as well as an MRI scan) to check the extent of my prostate cancer, I was given an injection of a radioactive substance and then told to go away for a few hours before returning for the scan. That type of injection is very simple and not really any different to, say, a Covid vaccination. (It was a novel experience though to realise that I was radioactive for a while, with instructions to avoid young children and pregnant women!).
With the CT scan I had prior to radiotherapy, I was injected with a contrast medium. Although it was called an 'injection', it was closer to a 'drip', as I had a canula inserted into my arm so that the contrast medium could be pumped into my body throughout the scan.
Neither was difficult or painful though and there were no side effects (apart, possibly, from remaining radioactive for a few hours after the first scan!).
So you (or whoever it is who's having the scan) should have nothing to worry about.