Evidence-appraisal glossary
Co-Intervention
Extra care or treatment, beyond the intervention being tested, that one trial group receives more than the other. When it differs between arms, it can manufacture or mask an apparent effect that is not due to the treatment itself.
Also called: concomitant treatment.
Co-interventions are a particular worry in unblinded trials, where knowing the assignment can lead clinicians to manage groups differently, for example adding rescue drugs or closer monitoring to one arm. The result is performance bias: the comparison is no longer clean because the groups now differ in more than the intended treatment. Blinding and clear rules for concomitant care are the main defenses.
Read the full Reading the Evidence blog.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.