Phases of Clinical Trials

Approved by the Cancer.Net Editorial Board, 09/2019


A full-text transcript is available.

Clinical trials are research studies that include human volunteers. Before they start a clinical trial, doctors must prove there is a chance that the new treatment will work better than the one available now. They do research until they can prove this. For example, they might test the treatment on laboratory animals. They do this to make sure it is safe to test in people.

What happens in different phases of clinical trials?

Once clinical trials are approved to start, each one must follow certain steps in order. The steps are called "phases." They are designed to keep volunteers safe. Making sure all the steps are done helps protect patients and give accurate results about what the clinical trial is testing.

You may join any phase of a clinical trial. The clinical trial just needs to be appropriate for you, your health, and your cancer. Here is a chart about the different phases of clinical trials.

Phase I clinical trials

Doctors do a phase I clinical trial to learn if a new drug, treatment, or treatment combination is safe for people. They may have already tested it in laboratory animals.

In a phase I clinical trial, doctors collect information on:

  • The dose or treatment

  • When you take it, and how often

  • Any side effects or problems

  • How the treatment affects you, such as how it affects the cancer or side effects

In a phase I clinical trial, you could be one of the first people to get the new drug or treatment.

Phase I clinical trials each last several months to a year. They usually have 10 to 30 volunteers. The treatment might help the cancer. Also, information from the clinical trial may help other people in the future.

Phase II clinical trials

A phase II clinical trial tells doctors more about how safe the treatment is and how well it works. Doctors also test whether a new treatment works for a specific cancer. They might measure the tumor, take blood samples, or check how well you can do certain activities. Or you might keep a log of your daily activities and symptoms. These are all ways to learn how well the treatment works.

A Phase II clinical trial lasts about 2 years. Volunteers sometimes receive different treatments. For example, a phase II trial could have 2 groups.

  • Group 1 – People who receive the usual treatment for the condition. This is also called the standard treatment. It is the best treatment known.

  • Group 2 – People who receive the usual treatment plus the new treatment doctors are studying.

Or a phase II clinical trial could have 3 groups. Volunteers in each group get a different dose of the treatment doctors are studying.

If the phase II clinical trial shows the treatment works and is as safe as the regular treatment, doctors can do a phase III trial.

How do doctors put volunteers into groups in a clinical trial?

Doctors use a computer program to put volunteers into different groups. The computer does this at random, which means by chance. Each volunteer has an equal chance of going in any of the groups. The name for this process is "randomization."

Using a computer to put volunteers in groups keeps the research staff from possibly changing the clinical trial results. They might do this if they chose who went in which group. For example, they might think a certain volunteer would benefit from the new treatment. So they might put that person in the new-treatment group. But this could change the clinical trial results. Randomization helps avoid this. It is very important to use randomization when a clinical trial compares 2 treatments or more.

Phase III clinical trials

A phase III clinical trial tests a treatment that worked well for volunteers in a phase II clinical trial. Doctors use phase III to compare the new treatment with the standard treatment. They want to know if the new treatment is better, has fewer side effects, or both. So they put volunteers in different groups. The volunteers in each group get a different treatment.

Phase III clinical trials can take many years. They may have several thousand volunteers. These must include men, women, and people of different ages and ethnic groups, if possible. This helps doctors learn how the treatment works in different people.

If a phase III clinical trial shows the treatment works well, doctors might begin using it with people outside the clinical trial. For example, if they learn that a certain amount of exercise lowers your cancer risk, they publish a report. This shares the information with other doctors. If the researchers or sponsor learn a new medicine is safe and effective, they can ask the government to approve it for people to use. In the United States, they ask the Food and Drug Administration (FDA). The FDA looks at the results of the clinical trial's phases. They approve the treatment if the results meet their standards.

Phase IV clinical trials

Doctors can prescribe a drug for their patients after the FDA approves it. But the FDA may require the sponsor to keep studying that approved treatment. In these clinical trials, doctors may check if the treatment benefits people as much as it did earlier. They also look for more possible side effects. These clinical trials are called phase IV clinical trials.

In a Phase IV clinical trial, doctors might study the drug or treatment in different doses, or with other drugs or treatments. Or they might study how it works if people take it at different times. They might study it in different people than earlier clinical trials did. For example, they might study how well it works for children or older adults. Doctors can also study how well a drug or treatment works over time.

Drug makers may do phase IV clinical trials even if the FDA does not ask them to. They might do this to get FDA approval to use the drug in a new way. For example, they might want to use it for another type of cancer.

Phase IV clinical trials can also check the safety of drugs or treatments being used now. They do this to make sure drug makers report any new or serious side effects. The FDA may take away a drug's approval if new research shows it is not as safe or effective as earlier testing showed. Doctors cannot prescribe it any longer if this happens.

How are clinical trial phases different from cancer stages?

It is easy to confuse cancer "stages" and clinical trial "phases." They use similar numbers. Clinical trial phases are numbered I, II, III, and IV (1, 2, 3, and 4). Cancer stages are 0, I, II, III, and IV (0, 1, 2, 3, and 4). But the numbers describe different things. A clinical trial's phase number tells you what doctors are testing in that phase. It also tells you how many volunteers are in the study. The stage of a person’s cancer tells you:

  • How much the cancer has grown and spread

  • What type of cancer cells are present. Some types mean the cancer is likely to get worse and some do not.

You can join any phase of a clinical trial with any stage of cancer, depending on the clinical trial's rules. The phase does not have to match your cancer stage. For example, you might join a phase II trial when you have stage IV cancer.

Do I need to be in all the phases of a clinical trial?

No. You can join any phase of a clinical trial if you qualify to join. For example, you may join a phase II clinical trial whether you were in phase I, or not.

Sometimes different phases are done at the same time. If so, the research staff will let you know. You always have a choice to be in the clinical trial, and you may leave at any time.

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Related Resources

Drug Discovery and Development

Finding a Clinical Trial

More Information

Clinicaltrials.gov