All I can add is a couple of things you may already be aware of:
NMDA is the abbreviation for a synthetic, non-natural compound, N-methyl-D-aspartate. That compound was found to be a strong antagonist of glutamate, the natural amino acid ligand which activates a certain type of Ca2+ channel, the NMDA receptor.

Are you finding the beneficial effects, meds kicking in AND induction of drowsiness, only when you take both DXM and benadryl?

Robert