Clinical drug investigation is a complex and long-lasting process demanding knowledge and large financial funds. The procedure of conducting a clinical trial is regulated by appropriate legislative regulations and international guidelines. The clinical trial of a new drug has been traditionally divided into four phases, and the first three ones are carried out before the drug approval from regulatory authorities. It is possible to design a clinical trial in different ways, but an ideal clinical trial today is considered to be that one which is controlled, randomized and double blind. The randomization and masking (blinding) strategies are considered to be the most important in avoiding bias when running, evaluating and interpreting the results of clinical trials. Two of the most frequent types of clinical trial design, concerning the therapeutic control, are a parallel group design and a crossover design. Clinical trials are essential for obtaining relevant data about a new drug compared with a control group, but also new data about the disorder being treated. This is a precondition for the approval of a new drug. The study conducted in accordance with the good clinical practice guidelines is therefore desirable not only for patients but also for physicians, i.e. for the medical level of a specific area which is so best included into the most recent global knowledge of medicine. Clinical trials are also of interest for the community because they are also useful from the financial aspect in saving money for treating specific disorders