Channeling bias exists in the dispensing of single-ingredient paracetamol versus ibuprofen. In previously published papers, models attempting to control for such channeling may not have adequately controlled for this bias.

Propensity score model diagnostics and negative control outcomes can be used to check the adequacy of models to control for channeling bias.

Large-scale propensity score matching can reduce channeling bias and should be considered for bias reduction in future observational studies in which the possibility of channeling bias is a major concern.