Dextromethorphan no better than a placebo...... but in what regard was not specified. i.e., for what effect ? DM certainly does have more than a placebo effect in some agencies, but has less than placebo in others. Just as do all "herbs", i.e., some having placebo effect on bronchodilation yet the same herb efficacious in reducing vasodilation.
DM is promoted as a cough suppressant. For example the product Robitussin DM. This is what it was found to be ineffective for. The abstract for the most current study can be found on Medline.