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What do the different phases of clinical trials mean?

Phase I Studies


Phase I clinical trial researchers are often testing a treatment which may be in the earliest stages of its development in a small group of healthy people (i.e. usually 10 to 30 volunteers). The information gathered from such trials will provide the manufacturers with important data on how the medicine is tolerated and how the body absorbs, breaks down and eliminates the drug and whether any side effects have been experienced by those taking part in the trial. The starting doses of the drug used in Phase I studies are very small and are only increased during the study if no significant side effects or intolerance are seen.

These trials are an important step in finding new treatments for the future and it is usually for these kind of studies that PAREXEL is looking for healthy volunteers.

Phase II Studies


Once a new medicine has successfully passed the Phase I study stage, the drug will be administered to a larger number of people (i.e. usually 100 to 200 patients). In these trials the drug will be tested in those who are actually suffering from the condition the medicine is intended to treat. The objective of Phase II studies is to help the manufacturer of the medicine to identify the medicines with a genuine therapeutic potential (the drug will actually help to treat the condition that it is being targeted towards). A drug that has successfully passed the Phase II stage has a 60% chance of actually being approved to go on to the market. PAREXEL actively recruits patients with various illnesses to conduct Phase II studies within the Harrow Unit.

Phase III studies


Phase III trials are an important turning point for the manufacturers of the new medicine. The purpose of phase III trials is to gather even more information about the effectiveness and safety of the new medicine from large numbers of patients (likely to be several hundred to several thousand patients), comparing the medicine’s effectiveness with standard treatments. Phase III studies generally run from two to four years and are likely to be carried out in many different centres – even different countries. At this point the new drug has better than a 70% chance of being approved and getting on to the market.

Phase IV Studies


Phase IV trials are carried out when the medicine has become available on the market either for prescription or is available over the counter. The information gathered from such trials provides the regulatory authorities and the pharmaceutical company with valuable information on the drug’s properties when administered to different groups of the population, and on any side effects that may become apparent with continued use.


The PAREXEL Clinical Pharmacology Research Unit
Level 7, Northwick Park Hospital, Watford Road, Harrow, Middlesex HA1 3UJ
VOLUNTEER FREEPHONE - 0800 389 8930 | Email: drugtrial@parexel.com