I use 16 mg orally (two tablets) at the same time I use the DXM (pure powder in a capsule). I started out using just 4 mg and noticed every time I used DXM with it was like the the best of my DXM trips in the past without it, so I tried a higher dose of ondansetron.
For me, the difference is pretty radical. The trip is far more lucid, euphoric, and down right manic. Combing it with LSD may have been the best experience of my life.
For two of the three others who have tried it the trip was substantially more lucid than DXM alone, but not substantially more euphoric or manic as it is for me. The last of the others who tried it reported no noticeable differences between it and a normal DXM dose. Like DXM by itself, the combo seems pretty finicky between people.
Ondansetron is broken down by the same enzyme that breaks down DXM to DXO, so the ratio of the two may be changed using high dose ondansetron (though according to Wikipedia it has little effect on other drugs broken down by the cytochrome P450 system, so maybe not -- then again 16 mg is a much higher dose than is normally prescribed). Additionally, the excess serotonin in the synapse as a result of DXM's reuptake inhibition is of course prevented from binding to 5HT-3 receptors, which are associated with anxiety reactions and nausea. Perhaps some of the dysphoric effects of DXM are mediated by serotonin binding to 5HT-3 receptors, and when this action is blocked in certain lucky individuals DXM becomes a much better drug. Those are the two known mechanisms I can think of to somewhat explain the presence of an interaction.
If you try it using ondansetron from India break the tablets before swallowing. The one time I didn't break them I got nauseated and had a "normal" DXM trip. I assume the tablets were manufactured hard to survive shipping and don't dissolve properly when swallowed whole (as I've read about some other Indian meds). Good luck. Let me know if it works for you.
Kamuix: I definitely relate to the serotonin high when using the combo of DXM and ondansetron (my vision gets really swimmy and of course there's a strong euphoria), but not from using the pure powder alone. I've only ever used pure powder so I can't comment on differences between it and syrup. What do you mean about the powder not being "mixed properly"? I don't mix it with anything.
