In a drug study, a placebo may resemble the medicine without containing the active ingredient being tested. Comparing outcomes between the groups helps researchers assess whether the drug adds a benefit beyond effects related to expectations and the context of care.
In randomised studies, people are assigned to groups by chance. This reduces the risk that decisions about who receives a treatment favour one group from the outset. Not all studies use a placebo, however: many compare a new treatment with an existing therapy.
Receiving a placebo does not necessarily mean receiving no care. In some cancer trials, for example, both groups receive standard treatment; one also receives the new drug, while the other receives a placebo. Any use of a placebo is explained during informed consent.