Knowing the experiment's purpose can alter participants' behavior. this is a major problem with

The answer to this question is informed consent.
informed consent. refers to giving the participants full knowledge about the structure of experiments before able to do it to them.
In most cases, this will make the partcipants alter their behavior so they could show positive traits during the experiments

Sometimes but itt depends, their is a lot if drugs on campus too so you really can't just say that