Phase I

Phase I, or early-phase clinical trials, test promising new agents in small numbers of patients and are critical to developing new cancer treatments and interventions. These initial trials set the stage for the larger trials needed to determine whether a drug is safe and effective. As cancer therapy becomes more precisely targeted to the unique molecular profile of a patient’s tumor, early-phase trials are taking on greater importance.  Phase I trials look at how an agent (drug) affects humans and what dose (amount of drug) is safe. 

Phase II

Phase II trials determine an agent's effectiveness on the cancer.  This phase provides additional evidence of biological effects against cancers. Usually, a group of 25 to 100 patients with the same type of cancer get the new treatment in a phase II study. They are treated using the dose and method found to be the safest and most effective in phase I studies.  In a typical phase II clinical trial, the volunteers usually get the same dose, but some phase II studies do randomly assign participants to different treatment groups, much like what’s done in phase III trials (see the next section). These groups may get different doses or get the treatment administered in different ways to see which provides the best balance of safety and effectiveness. Along with watching for responses, the research team keeps looking for any side effects to record. If enough patients benefit from the treatment and the side effects aren’t too bad, the treatment is allowed to go on to a phase III clinical trial.  .

Phase III

Treatments that have been shown to work in phase II studies usually must succeed in one more stage of testing before they are approved for general use. Phase III clinical trials compare the safety and effectiveness of the new treatment against the current standard treatment.  Because doctors do not yet know which treatment is better, study participants are often picked at random (called randomized) to get either the standard treatment or the new treatment. When possible, neither the doctor nor the patient knows which of the treatments the patient is getting. This type of study is called a double-blind study.  Most phase III clinical trials have a large number of patients, at least several hundred. These studies are often done in many places across the country and are more likely to be offered by community-based oncologists. These studies tend to last longer than phases I and II and placebos may be used in some phase III studies.

Other Clinical Trial Info

People interested in taking part in a clinical trial should talk with their doctor. General information about clinical trials and where they are offered is available from the NCI's Cancer Information Service (CIS)  at, or you may call 1-800-4-CANCER.