Phase II of clinical trials involves assessment of the study drug efficacy in a predetermined group of patients, and the level of safety for the patients.

The assessment includes a correlation between the dose and potential effect of the study drug, which results in determination of an adequate dose of the substance to be used in subsequent trial phases. During the entire stage two, the study drug is assessed with regard to its safety; a detailed analysis is applied to data on absorption, metabolism and excretion of the drug, depending on sex and age. At this stage, the effect of the new drug is compared either with placebo or with the drug used so far in the treatment of a specific disease entity. Comparisons are made with the use of “blinded” method, which ensures the most objective assessment of the effect. In practice, this means that neither the patient nor the investigator knows if the patient receives the study substance or placebo. The group participating in this stage is randomised and includes several hundred volunteers, including especially those with the diagnosed disease entity for which methods of effective therapy are searched. A positive assessment of Phase II is achieved when the level of benefits versus the risk of using the substance is significantly higher.

Phase III of clinical trials involves final confirmation of the efficacy of a given substance in the treatment of a specific disease entity.

The last but one stage of clinical trials involves determination of a relationship between safety of the drug and its efficacy in short- and long-term use. The studies include a group reaching a few thousand patients, and the duration is between one year and several years. Similarly to phase II, this phase also involves double-blinded methods, and random selection of patients. After positive completion of phase III, the drug may be registered and granted marketing authorisation, and the documentation related to the drug is submitted to an institution registering the medicinal product. A complete set of documents, whose contents is consulted in detail with national drug agencies, may include even several thousand pages and has to include all the data collected in the period of preclinical and clinical studies of phases I to III.

Phase IV refers to drugs already registered and marketed; its purpose is to determine the level of safety for all indications recommended by the manufacturer and for all indicated groups of patients. During the last phase, the results from previous stages are once again verified and new indications for the use of the already registered drug are prepared.

Patient participation in each stage of clinical trials is possible after detailed medical verification, and requires voluntary signing of a so called Informed Consent Form