Personally i feel/think and has been my experience that DMT's Adrenergic properties is what can trigger nausea/vomiting (whether orally or smoked via Changa or with oral Harmalas), but kinda like with some of the Harmala-related side-effects, it seems at least ime that once you get used to the Adrenergic effects of DMT the more undesirable side-effects will go away, like the nausea/vomiting.
When it comes to oral Harmalas+oral DMT, ime most of the nausea/vomiting/purgative effects seem to come from the Harmalas, even if there's no nausea/vomiting for someone from the Harmalas alone, and then DMT is added and the nausea/vomiting happens, but to me it seems like the DMT triggers the Harmala's purgative effects and thus brings out the Harmala purge, and so with oral DMT at least a lot of the purging can be dealt with by regularly consuming Harmalas for a few weeks or so which will build up the Harmala reverse tolerance while desensitizing the properties that cause bodyload and nausea/vomiting and such, then DMT can be added to the mix in a much cleaner way without the Harmalas' purgative effects getting in the way. But, if one still experiences nausea/vomiting even though one has consumed Harmalas regularly (in good dosages) for some time, then that to me says it's the Adrenergic properties, which if you then consume the DMT regularly for a little bit, the DMT-related side-effects will be reduced while maintaining active DMT effects.
If the nausea/vomiting when it comes to DMT is indeed Adrenergic-related, then imo one could try testing out an Adrenergic Alpha 1 blocker that should not only knock out DMT's Adrenergic-related nausea/vomiting, but would also knock out the Adrenergic intensity. As far as the Serotonin 3 receptor goes, i haven't come across any studies that have specifically documented activation of the Serotonin 3 receptor by DMT or by Harmalas, though if one does know of a study that does feel free to share because i'm curious. With that said though, i've tried a few Serotonin 3 antagonists, like Zofran, Ginger, and Peppermint, and while some seem to get benefit from Ginger at least from what i've read, ime both Zofran and Ginger don't really seem to help with the nausea/vomiting and plus they make things really weird and like they alter the Aya in a weird undesirable way, maybe i used too much Ginger in my experimentations but i used common dosages iirc and i tried Ginger tea, Ginger root chews, Ginger essential oil, and the Zofran i know i definitely took too much of because i took like 8mgs before i learned that Harmalas inhibit CYP1A2 and CYP2D6 which Zofran is metabolized by so who knows how much Zofran i was actually consuming, but ime both Zofran and Ginger had a similar effect imo owing to their Serotonin 3 antagonism.
The only thing that really/truly helped me completely counteract the Harmala and DMT-related nausea/vomiting, was 8 to 10 drops of Limonene in a capsule taken at the same time as the Harmalas, and i'm not sure if it's because it gets digestion going, or if it's because it's a Serotonin 1A agonist which can inhibit the NK1 receptor and thus can be anti-emetic potentially, or if it's because it's an Adenosine A2A agonist which can crosstalk and interact with other receptor sites like reducing Dopamine 2 receptor activity (which Dopamine 2 receptor stimulation apparently can also cause nausea/vomiting, and so reducing Dopamine 2 activity may counteract that), but ime/imo Limonene helps either due to being a Serotonin 1A agonist and thus an NK1 receptor inhibitor, or because it gets digestion going, but i think the Serotonin 1A agonism may play a big role.
) and didn't want to repeat that with my first DMT experience. I take an over-the-counter travel sickness medicine now for those times 
i'll have a bucket in standby-mode just in case, but sure it'll be fine!!
