In some clinical trials, participants may receive a placebo. A placebo is a pill or other treatment that looks like the study medicine or is used in the same way as the study treatment, but has no effect. The placebo is used in a study when the treatment team wants to compare what happens to the participants who use the study medicine or treatment to the participants who receive the placebo.

If the participants in the study have a serious illness that could get worse from taking a placebo (meaning not taking the real medicine), placebos are not used. Instead, a medicine or treatment that is different and known to be effective for the illness is used. In general, all participants are told before they decide join a clinical trial/study if a placebo might be used.


Clinical trials must avoid bias to be sure what is being studied is accurate. In research, bias is defined as: “a systematic error introduced into sampling or testing by selecting or encouraging one outcome or answer over others” (Merriam-Webster dictionary). In clinical trials, often either the participant, or both the participant and researcher don’t know what type of medicine or treatment the participant is taking. This is done to decrease bias as much as possible.

For example, a participant might be biased toward a medicine and think that it is working if they are told they are taking a new medicine that cures their disease. Just believing that you are taking a medicine to cure the disease might make you think or feel like you are getting better, even if you are not.

Single-blind (masked) study

In this type of trial, the researchers know which medicines they gave the participants, but the participants do not know what they were given. This method is used to decrease bias by participants.

Double-blind (masked) study

In this type of trial, only the pharmacist knows what medicine or treatment the participant is getting. This method is used to decrease bias by the participants and the researchers. If there is ever a problem during treatment, like if the participant gets sick, the pharmacist will stop the study and will make sure the participant gets proper treatment.

For more terms and phrases relating to clinical trials, visit the NIH Glossary of Common Terms.