An active placebo is a placebo that produces noticeable side effects. This is in contrast with an inert placebo, which has no side effects. The purpose of an active placebo is to reduce unblinding in blinded experiments.
Relevance to clinical research
When a treatment has side effects while the control does not, patients who experiences side effects may correctly guess that they are receiving treatment. If patients are able to discriminate between treatment and placebo at a rate greater than chance, the experiment is not truly blind. This introduces bias in favor of the treatment group and may lead to a type I error.