In phase I trials, new drugs or new treatments are tested for the first time in a small group of healthy volunteers. This involves checking how the drug acts on the human organism (pharmacodynamics) and how it is absorbed, distributed, metabolized, degraded and excreted in the human body (pharmacokinetics). This determines the effective dosage at which the lowest possible side effects occur. The information collected on the safety and tolerability of a new drug is the basis for conducting a phase II trial. This can only be done if the side effects are not too severe.
In phase II trials, new drugs are administered for the first time to patients suffering from the target disease. The goal of this phase is to determine a dose that will show the desired therapeutic effect while minimizing side effects. Phase III studies will not be approved until Phase II studies can demonstrate that side effects are acceptable.
Phase III trials are conducted in a larger group of patients and are designed to confirm the safety, efficacy and safety of a drug. This phase runs under real-world conditions and is also designed to identify rare side effects, drug-drug interactions and optimal dosing. In this phase, the new drug is often compared in a comparative study against previous standard therapies or placebo. Thanks to intensive medical care, the risk for participating patients remains low.
Phase IV trials are conducted after a new drug is approved. They are used to gather additional information on the efficacy and safety as well as long-term risks of a drug and are therefore performed in large patient groups. This means that even very rare side effects can be recorded.