A placebo is a substance given to a study participant
that has no known treatment value. Placebos will be made as similar to the actual treatment as possible. If the new medication being studied is in the form of an injection, the placebo will be an injection. The idea is that the study participant and the researchers do not know who is getting the real treatment and who is getting a placebo.
However, if the medication being studied is for a condition where a medication already exists, that medication will often be used in the control group, rather than a placebo. In MS clinical trials, placebos are often not used because effective medication exists.