A medicine given to every study group cannot explain the randomized difference by itself.
A pediatric trial compared caudal local-anesthetic regimens, including a fentanyl-containing group. All children received oral midazolam before surgery. The investigators reported similar sedation scores across the randomized groups.
The trial did not randomize midazolam exposure, so it cannot estimate a midazolam-versus-no-midazolam effect. Similar scores across the caudal regimens do not establish absence of effects from the shared premedication.
Read the complete anesthetic protocol before citing an adverse-event or sedation comparison. Separate the randomized intervention from background treatment. In a case reconstruction, the shared exposure still belongs on the timeline even though the trial cannot isolate its contribution.
Source: PMC4085267, supplied full-text paper. Findings are paraphrased; case-review implications are an interpretation.
Related: Forensic Toxicology: expert analysis and case review.