This is important for both the study participant and the researchers.
A placebo effect is a natural phenomenon in which a drug is said to have no effect when used in the absence of a treatment. In drug testing, it is used because the drug is believed to be ineffective when used by itself. But because people are often more willing to use a drug if they do not know they are taking it, it is sometimes also used to control for people’s own behavior when the drug is used.
If you are a new drug user, in order to be included in the new study, you will be given the drug you seek, and the researcher will do a placebo experiment for you. They will control for your current drug usage by using a placebo. This is done to guarantee the drug you are seeking has no effect on you.
This is why most people who want to be included in a new study will go to their doctor and get the drug they want. So if you know you need to get the drug, but you are not a treatment-seeking person, you can usually get the drug on your own.