Drug trials are called Phase 1, Phase 2 or Phase 3.
Phase 1 trials only study safety. More trials cannot take place unless Phase I trials show a drug to be reasonably safe when given to humans. People in a Phase I trial are closely watched for any sign of harm caused by the drug. Because safety is the question, Phase 1 trials are small but they also allow researchers to start understanding how the drug will work in people. At this point in testing, no one even knows if the new drug will do what it is designed to do. Phase I trials are the most dangerous for patients.
Phase 2 trials confirm that a new drug is safe enough for more people to try, and show whether a drug is effective over the short-term. Phase 2 trials also help decide what doses will be used in Phase 3 trials. Phase 2 trials are usually placebo-controlled and double-blinded. Neither the patient nor the doctors know if a patient is getting the drug or a placebo . These trials are larger than Phase I trials and take longer. There is still significant risk because only a small group of people has tested the drug for safety so far.
Phase 3 trials prove or disprove the effectiveness and safety of a drug over the "longer" term. They are usually double-blinded and placebo-controlled. They can involve hundreds or thousands of patients over months or years. Although the number of patients can be large, risks for patients are smaller because of all the earlier testing. The large scope of these trials lets researchers observe benefits and overall safety of the drug as well as identify less common side effects. Phase 3 trials are incredibly expensive and only drugs which look very profitable make it to this type testing.