Phase 1 Studies: In a Phase 1 study, humans are given a new drug for the first time. Usually, healthy people are enrolled in Phase 1 studies. Sometimes people with a certain disease may also be enrolled. Phase 1 studies help find out how a drug is broken down in the human body and how it interacts with the human body. Phase 1 studies reveal some of the side effects associated with a new drug. Information from Phase 1 studies is used to develop Phase 2 studies. Phase 1 studies can involve two groups, one group of people who get the new drug and another group which gets an inactive sugar-pill (placebo). About 20 to 80 people are included in a typical Phase 1 study, which lasts on the order of days to weeks.

Phase 2 Studies: In a Phase 2 study, a new drug is tested to see if it helps improve a particular illness. Researchers give the drug to people with the disease under study at the doses, and on the schedule, found to be safe in Phase 1 trials. During Phase 2, researchers collect additional data on the safety and efficacy of a drug, including short-term side effects and risks, and collect additional information about the proper dose and dosing schedule. Phase 2 studies often have at least two groups, people who get the new drug and people who get placebo; sometimes a standard treatment is also included for comparison. About 100 to 250 people are included in a typical Phase 2 study, which lasts on the order of weeks.

Phase 3 Studies: In a Phase 3 study, research is expanded to longer, larger trials, after Phase 1 and Phase 2 studies have shown that the risk of side effects from a new drug is generally outweighed by its being helpful for the illness under study. However, since Phase 1 and Phase 2 studies include only a few hundred patients total, Phase 3 studies gather additional information needed to evaluate the overall benefit-risk relationship of a drug. Researchers collect additional information about drug-related side effects, including less common side effects. Several hundred to several thousand participants may be enrolled. Phase 3 studies commonly have at least two groups, people who get the new drug and people who get placebo, as well as a standard treatment group included for comparison. At the conclusion of a properly designed Phase 3 trial, the new drug will be found to be inferior, equivalent, or superior to placebo and/or the standard treatment. About 300 to 3,000 people are included in a typical Phase 3 study, which lasts on the order of weeks to months.

Phase 4 Studies: The intent of Phase 4, or post-marketing studies, is to learn more about the drug after it has been approved by government authorities. In Phase 4 studies, researchers gather information about an approved drug's risks, benefits, and best uses in ‘real-life’ conditions. These studies can also include trials of different doses or schedules of administration, other stages of disease or other disease and age groups, cost studies, quality-of-life studies, or use of the drug over a longer period of time.

Blinded Studies: In a single-blind trial, the researcher knows the details of the treatment but the patient does not. Because the patient does not know which treatment is being administered (the new treatment or another treatment) there should be no placebo effect. In practice, since the researcher knows, it is possible for them to treat the patient differently or to subconsciously hint to the patient important treatment-related details, thus influencing the outcome of the study.

Double Blinded Studies: In a double-blind trial, one researcher allocates a series of numbers to 'new treatment' or 'old treatment'. The second researcher is told the numbers, but not what they have been allocated to. Since the second researcher does not know, they cannot possibly tell the patient, directly or otherwise, and cannot give in to patient pressure to give them the new treatment. In this system, there is also often a more realistic distribution of sexes and ages of patients. Therefore double-blind (or randomized) trials are preferred, as they tend to give the most accurate results.