Clinical drug trials are usually divided into three phases. In the first phase, researchers work with a small group of people to find out how safe a new drug is. They are also concerned with finding out how best to deliver the drug and the optimal dosage. This then leads to the second phase where investigators give the drug to a larger number of patients. In this phase of clinical drug trials, the researchers continue to monitor the safety of the drug and whether it works.
To ascertain that the drug works, researchers have to look for information that shows it has a positive effect at all. It could be something like whether it decreases a blood marker associated with the disease. When this happens, the clinical drug trials can move to phase three.
At the third phase, the drugs are tested for safety and efficacy in patients in the hundreds or thousands. A groups of patients are given the drug while others are treated with a placebo or standard care therapy. Clinical drug trials at this stage involves comparison of the group on the drug with those on the standard therapy or placebo. Although rigorous clinical trials are done to ensure the drug works, it could all be a matter of chance. However, in most cases, clinical trials and comparator sourcing does work.