Study phase

Phase I Trials
These are initial studies to determine the metabolism and pharmacologic actions of drugs in humans (what are called PK/PD studies or pharmacokinetic / pharmacodynamic studies), the side effects associated with increasing doses (a dose response curve or dose response relationship), and to gain early evidence of effectiveness; may include healthy participants and/or patients. These are sometimes called a first in man study.

Phase II Trials
Controlled clinical studies conducted to evaluate the effectiveness of the drug for a particular indication or indications in patients with the disease or condition under study and to determine the common short-term side effects and risks. Sometimes multiple doses will be studied in these trials to select the appropriate dose for further testing in phase III. These are sometimes called a dose finding study.

Phase III Trials
Expanded controlled and uncontrolled trials after preliminary evidence suggesting effectiveness  of the drug has been obtained, and are intended to gather additional information  to evaluate the overall benefit-risk relationship of the drug and provide and adequate basis for physician labeling. These are also called pivotal trials.

Phase IV Trials
After the drug has been approved by a regulatory agency, these are post-marketing studies to delineate additional information including the drug's risks,benefits, and optimal use in a broader group of real world patients.