Phase III studies are only conducted when the results of Phase II studies, the exploratory therapeutic studies, have shown that the new drug is safe and can be effective in a sample of patients. Phase III studies are also called confirmatory clinical studies or validation clinical studies: they test whether the results established in Phase II can be confirmed. 

In general, Phase III studies are large-scale studies with thousands of patients, conducted at different sites, sometimes in different countries. However, if the drug candidate is intended to treat a rare disease, the number of participants will naturally be smaller.  

The objective of Phase III studies is to confirm that the drug candidate is effective and safe in a larger group of patients, and to reduce the side effects observed in the previous phases.  

As in Phase II studies, the drug candidate is compared to a placebo or, if available, to an existing treatment (usually the standard of care). For this purpose, phase III studies are usually randomized and it is also common to conduct them in a (double)blind manner.  

The results of phase III studies are an essential part of the dossier that must be submitted to the regulatory authorities to request authorization for the drug candidate to be marketed. 

Phase III studies last from one to several years.