In Phase II, one hundred to three hundred subjects with the targeted disease are selected. These patients cannot have other medical conditions that might complicate the assessment. For example, if a drug is being tested to reduce blood pressure, the subject cannot have high blood pressure and diabetes.
Phase II trials are randomized, double-blind, placebo-controlled, clinical trials. Randomization is the process of assigning the subjects in the trial into two groups, the control group and the treatment group, in a random manner. This helps to eliminate bias when assigning patients to each group. Double-blind means that neither the subjects nor the people giving the drug know to which group each subject has been assigned. Placebo-controlled means that the control group receives a placebo—a pill that does not contain the drug—while the test group receives a pill with the drug. All these provisions help to eliminate bias that may skew the results of the trial.
Phase II clinical trials are several months to two years in duration. They are carried out to determine the best dosage for the drug, look for unwanted effects (adverse effects), and to evaluate the efficacy of the drug; that is, whether it is useful to treat the target disease.