I tend to almost always take ondansetron (4mg) alongside any oral psychedelic these days; the synergy is excellent. It selectively antagonises the SERT receptor responsible for some pathologies behind Serotonin (SERT)-induced nausea/vomiting/purgation. IME, 4mg of ondansetron coadministered with a nauseating psychedelic (e.g. mescaline, LSD/MDMA, psilocybin mushrooms, etc) almost entirely eliminates the nausea whilst not affecting the trip itself whatsoever.
I have not tried combining it with changa (AND given MAOI drug interactions, I wouldn't suggest using harmalas with it at all; If I take ondansetron with a psychedelic, I don't take harmalas, and vice-a-versa.)
It's a godsend for me; but please remember: One Man's Medicine is another Man's Murderer.
Stay Safe,
-Godsmacker
'"ALAS,"said the mouse, "the world is growing smaller every day. At the
beginning it was so big that I was afraid, I kept running and running, and I was glad
when at last I saw walls far away to the right and left, but these long walls have
narrowed so quickly that I am in the last chamber already, and there in the corner
stands the trap that I must run into." "You only need to change your direction," said
the cat, and ate it up.' --Franz Kafka