​The main steps that are taken to maximize the safety of patients in a phase I trial are as follows: A low starting dose is selected, based on observations in animals given the drug. Each patient is carefully observed for side effects, and new patients are only entered into the trial if the dose appears safe. The dose of treatment being studied is carefully increased according to a predetermined plan, again with close observation at all times of patients at each dose. Once patients do begin to experience more difficult side effects of a treatment, the trial stops and may go on to phase II testing with a slightly lower dose. Patients must have fairly normal heart, liver, lung, nerve, and kidney function, so that there is the least chance of harm from the new treatment. Fairly extensive and close monitoring is done for patients throughout their treatment on the study, to ensure that side effects of the treatment are detected. Patients are usually required to live reasonably close to the cancer treatment centre, so that if any side effects do develop, they can be assessed very carefully and promptly.

At this stage of testing, due to the need to emphasize safety for patients, the investigators are not primarily testing the usefulness or effectiveness of a new treatment; rather they are focussed on determining whether it will be safe to go on to further testing in larger numbers of patients. It is possible that a patient receiving treatment in a Phase I trial will experience a good response to a new drug, such as a shrinkage of their cancer, but the chance is quite small. The patients taking part in these trials may have a wide variety of different types of cancer.