DXM is a non-competitive blocker. This may be a biogenic amine reuptake complex. If so, then these class of chemicals may act as reuptake inhibitors. The role of the PCP2 site is poorly understood. All of these drugs are being studied for their effects in preventing damage to the brain during siezure.
Nausea can occur.
DXM has some stimulant effects. Robitussin DM also contains an expectorant which should not be taken in high doses. The usual warnings apply. Additionally, prolonged use of DXM can and has led to psychosis similar to PCP-induced psychosis. Individual differences in NMDA receptors may be at work here, but you're still potentially at risk.