What is the term for a substance containing no active ingredient often used in clinical trials?

The term for a substance containing no active ingredient that is often used in clinical trials is "placebo." Placebos are crucial in research, particularly in clinical trials, as they provide a control against which the effects of an active drug can be measured. In such trials, participants may be given a placebo instead of the actual treatment to help determine the efficacy of the drug being studied. The results can indicate how much of the drug's effect is real versus how much may be due to participants' expectations or psychological factors.

Placebos are particularly valuable in double-blind studies, where neither the participants nor the researchers know who is receiving the active treatment versus the placebo. This approach helps eliminate bias and allows for a clearer understanding of how effective a treatment may be.

The other terms listed, such as "adjuvant," refer to substances that enhance the immune response in vaccines or cancer treatments, "allele" pertains to variations of a gene, and "antigen" refers to a substance that induces an immune response. These concepts do not pertain to the use of a non-active treatment in trials and are therefore not correct in this context.