DXM, as the hydrobromide salt, is absorbed quickly from the GI tract; within 30 minutes, all of it may have entered the bloodstream (2,3). The polistirex compound is intended for continuous absorption, and may take 6 to 8 hours to fully enter the bloodstream.
DXM is metabolized via two pathways, both of which lead to the same thing, 3-hydroxymorphinan (3HM). The first pathway goes from DXM to DXO (dextrorphan) and then to 3HM; the second goes from DXO to 3-methoxymorphinan (3MM) and then to 3HM. By far most of the DXM (up to 90% ) gets metabolized via DXO in normal individuals.
DXM is converted to DXO by a liver enzyme called cytochrome P450-2D6 (debrisoquine 4-hydroxylase). About 7% of Caucasians and 0.5% of Asians have a highly inefficient (70 times slower) version of this enzyme, and cannot metabolize DXM to DXO effectively (10). About 0.5 to 2% of the population have multiple copies of the P450 gene, leading to extremely fast metabolism of DXM into DXO (155). After being converted to DXO, the enzymes P450-3A4 and P450-3A5 convert DXO to 3-hydroxymorphinan (77).
The other pathway goes to 3-methoxymorphinan first (via P450-3A4 and P450-3A5), and then to 3-hydroxymorphinan. Most people do not metabolize much DXM this way, although people who lack the normal P450-2D6 will convert a substantial amount to 3MM. As 3MM is probably not psychoactive, this means that the 5-10% who lack the normal 2D6 enzyme will experience less effect from DXM (or more specifically, won't experience the effects of DXO).
P450-2D6 functions by removing the methoxy group and replacing it with a hydroxyl (OH) (or more accurately, pruning the methyl off the oxygen); this step is known as O-demethylation. P450-3A4 and 3A5 replace the methyl group with a hydrogen (H); this is the N-demethylation step. Refer to the diagram of the DXM molecule in Section 4.2 for the location of the methyl and methoxy groups.