dextrorphan

Dextrorphan (abbreviated DXO) is a dissociative anaesthetic used to block NMDA receptors. It may be administered following stroke to reduce metabolism and prevent ischemic brain damage. It is also used in to detect PCP receptors.

Non-medical drug users will encounter DXO as the primary human metabolite of dextromethorphan, or DXM. It is created when the liver enzyme cytochrome P450 (also called CYP2D6 or debrisoquine 4-hydroxylase) removes the methyl group at position 6 of the DXM molecule.

The quantities of DXM found in a regular dose of cough medication (~15mg) evidently do not produce active quantities of DXO. However, when DXM is consumed in recreational or psychedelic doses (100mg and up), large amounts of DXO are also created. DXO has dissociative and tranquilizing effects, which some liken to those of marijuana. These effects are thought to soften some of the more bizarre and intense effects of DXM, and indeed people who have low levels of cytochrome P450 tend to have more disturbing (and longer) trips.

The amount of DXO relative to DXM can be decreased by dividing one's dose (because another DXM metabolite, 3-methoxymorphinan, blocks cytochrome P450), or by injecting DXM subcutaneously.

All information except the first paragraph extracted from William White's DXM FAQ, the most impressive piece of lay scholarship I've ever seen.

It should also be noted that dextrorphan, according to William White is responsible for the stoned feeling of DXM trips, while the dextromethorphan itself is responsible for the more psychedelic aspects. Mr. White seems to be undecided about this, however.

Dividing the doses, or preloading, or just a metabolic fluke can lead some DXM trips to be exuberant and psychedelic, while others are stony and dizzy. This could be because of the varying effects of DXM and DXO, or it could just be that drugs that mess with Glutamate ain't nothing to mess with, the great number of Winamp visualization devices notwithstanding.