The steps taken to maximize the safety at this level of testing are as follows: The dose is carefully selected from information learned in the phase I tests. Patients must again have quite good lung, heart, liver, nerve, and kidney function. Patients again are expected to stay close to the treatment centre, so that they can be seen quickly if any side effects occur. Extra tests are done throughout the treatment period to watch carefully for possible side effects of the treatment. Close testing of the cancer is done to determine as quickly as possible whether a treatment is helpful to a patient or not. Often patients are only allowed to have had one or two previous types of cancer treatment drugs, so that there is the best chance that they may benefit from a new treatment, and so that there is the least chance that they will be too unwell to participate safely in the study.

Because this phase of clinical research is designed to test the effectiveness of a new drug, patients usually have to have cancer that can be repeatedly measured in some way, usually on a CAT scan (a special type of x-ray image) or other test. The number of patients treated in the study may depend on how many responses (shrinkages in tumour measurements) are seen in the first several patients. There may be multiple Phase II trials going on at the same time, examining the same new drug or treatment in different types of cancer.