Currently, some Phase II and most Phase III drug trials are designed as randomized, double blind, and placebo-controlled.

Randomized
A randomized controlled trial is the study design that can provide the most compelling evidence that the study treatment causes the expected effect on human health. Each study subject is randomly assigned to receive either the study treatment or a placebo.

Blind
The subjects involved in the study do not know which study treatment they receive. If the study is double-blind, the researchers also do not know which treatment is being given to any given subject. This 'blinding' is to prevent biases, since if a physician knew which patient was getting the study treatment and which patient was getting the placebo, he/she might be tempted to give the (presumably helpful) study drug to a patient who could more easily benefit from it. In addition, a physician might give extra care to only the patients who receive the placebos to compensate for their ineffectiveness. A form of double-blind study called a "double-dummy" design allows additional insurance against bias or placebo effect. In this kind of study, all patients are given both placebo and active doses in alternating periods of time during the study.

Placebo-controlled
The use of a placebo allows the researchers to isolate the effect of the study treatment. A placebo is an inactive pill, liquid, or powder that has no treatment value. In clinical trials, experimental treatments are often compared with placebos to assess the experimental treatment's effectiveness. In some studies, the participants in the control group will receive a placebo instead of an active drug or experimental treatment.

Although the term "clinical trials" is most commonly associated with the large, randomized studies typical of Phase III, many clinical trials are small. They may be "sponsored" by single physicians or a small group of physicians, and are designed to test simple questions. In the field of rare diseases, like gene therapy treatments of inherited disorders, sometimes the number of patients might be the limiting factor for a clinical trial.