In my subjective experience, Dextromethorphan (DXM, as the robo-trippers call it) does almost nothing for me, in the 1-5% range

The only cold and cough medicine that really truly works is the over-the-counter stuff, pseudoephedrine, works amazing for me. I usually pick up a box of the stuff when school starts in the fall and I go through half a box of it by the following summer.