In this study, a placebo is not used on the control group. The experimental group gets dextromethorphan while the control group gets "no treatment". This results in procedure bias, as subjects in different groups are not treated the same. Blinding and use of a placebo reduce the influence of participants and researchers on procedures and interpretation of outcomes as neither are aware of group assignments.
See table on bias and study errors in FA 2019 p 260.