Patients often receive multiple treatments, in addition to the intervention being studied, which could impact the outcome of interest. Co-intervention bias occurs when different groups receive different co-interventions.
For example, in a randomized, controlled trial studying haloperidol in gastroparesis, a number of alternative anti-emetics and analgesics, such as morphine and ondansetron, were also used, and the amount of these co-interventions was different between the two groups. (Roldan 2017) As a result, it is unclear whether the demonstrated reduction in pain and vomiting was solely due to haloperidol, other whether the other medications explain the difference.
Co-intervention bias is a type of intervention bias.
Roldan CJ, Chambers KA, Paniagua L, Patel S, Cardenas-Turanzas M, Chathampally Y. Randomized Controlled Double-blind Trial Comparing Haloperidol Combined With Conventional Therapy to Conventional Therapy Alone in Patients With Symptomatic Gastroparesis. Academic emergency medicine. 2017. PMID: 28646590 [full text]