Different Types of Clinical Trials

Clinical trials are conducted in a number of stages. Early-stage trials usually involve a small number of patients or healthy people. When psychological treatments or educational programs are being tested, these early-stage studies can be used to refine the treatment before it is assessed in a large group of people. For trials of medicines and other treatments, early-stage studies are carried out in a small group of people to assess safety by looking for unwanted side effects. Later-stage clinical trials typically involve larger numbers of participants and often use randomized trials.

A good example of how the clinical trial process helps to answer important questions is the development of new drugs. These are first developed in the laboratory to see whether they may be helpful in the prevention or treatment of a particular illness. They are then assessed in animals to check their safety and to find out how they affect the body. If they look like they may be of benefit and are likely to be acceptably safe, they will then be tested through various stages of clinical trials. For drugs, there are four stages.    

Phase 1 (early stage)

Phase 1 is the first stage and usually involves small groups of healthy people or sometimes patients. Phase 1 trials are aimed mainly at finding out how safe a drug is and will detect only common side effect.

Phase 2  

By the time a drug reaches Phase 2, researchers will know quite a lot about it and will detect less common side effects. Phase 2 trials aim to  

  • test the new drug in a larger group of people to better measure the safety and side effects and  

  • see if the drug has a positive effect in patients.  

Phase 3  (later stage)

Phase 3 trials are large and may include hundreds, or sometimes many thousands, of patients from all over the US, and often from several countries. The goal of phase 3 trials is to

  • compare the effects of newer drugs with the standard treatment if there is one,

  • find out how well the drug works and how long the effects last,

  • accurately assess safety and risks, and

  • find out more about how common and serious any side effects or risks are and about any possible longer-term problems that could develop.  

Phase 4  

Phase 4 trials are carried out after a new drug has been shown to work and has been given a license. Phase 4 trials are to learn 

  • how well the drug works when it is used more widely,

  • the long-term risks and benefits, and

  • more about the possible rare side effects.  

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