A placebo is a drug given to the patient to treat rather than to study any physiological effect. When a placebo is selected for study, then neither the doctors nor the patient knows that the medication or drug is being administered to the patient. A placebo is designed when the volunteer is provided with the drug, but no physiological effect is being monitored.
This method is usually done when testing is done for anti-depressants. Certain issues need to be considered while designing a placebo. For example, if asthmatic patients are on a placebo, and is attacked by a severe asthmatic attack, it is unethical to give a placebo.
In such cases, the volunteer must be immediately given the conventional drug, to ease out his symptoms. In cases of patients having life threatening conditions like AIDS (acquired immune deficiency syndrome) and cancer is not allowed to test the drug for placebo volunteers.