Researchers carry out treatment trials in a series of phases (or steps). They test possible new cancer drugs or treatments in a laboratory first.
To make sure drugs are safe, researchers test them on animals before they give them to people in trials.
If the drugs are effective in laboratory tests, researchers test them in phase 1 trials with people affected by cancer.
If these are successful, researchers test the drugs in phase 2 trials and then phase 3 trials.
Phase 4 trials test drugs that are already licensed for use.