Before a clinical trial, preclinical studies are carried out. These are the first tests to study the effectiveness and safety of a new treatment, both in cells and in experimental animals. If positive results are obtained, clinical trials can begin on humans, which are classified into four phases.

• In Phase I the safety of the medication, optimal dose and best means of administering the treatment are evaluated. This is carried out with a small number of participants.
• Phase II analyses the effectiveness of treatment, not just safety. There will usually be between 100 and 300 people participating.
• Phase III allows the effectiveness and safety of the treatment to be validated in a higher number of subjects. During this phase, the study treatment is compared with the current standard of treatment, should there be one. To make the results rigorous and objective, patients are usually randomly subdivided into two or more groups. One of the groups receives the new drug that is intended to be studied. The control group, on the other hand, receives a placebo (an inert substance) or a reference treatment. Whenever possible, patients and researchers do not know which group patients are in until the study ends, to avoid bias when analysing the results.
• If good results are obtained, the medication can be approved for marketing, though it continues to be studied in the long term. This is known as Phase IV and its purpose is to detect, in clinical practice, uncommon adverse effects that may not have been detected in previous phases.