This is very weird.
"We" -- not me -- have looked over the binding profile of 25x-NBOMe several times. Long story short, it binds 5-ht2a almost exclusively, 5-ht2c 10x less, mu1 100x less, 5-ht6 100x less:
The ligand had low affinity for most receptors, with the following reported Ki values (nM) for receptors where it had significant affinity: 5-HT2C (2); 5HT6 (73 ± 12); μ opiate (82 ± 14); H1 (189 ± 35); 5-HT2B (231 ± 73); kappa opiate (288 ± 50)
5-ht2a affinity is subnanomolar, somewhere around 0.3 nM.
As it stands the only thing separating the NBOMe drugs from LSD w.r.t action is: LSD has affinity also at 5-ht1a, 5-ht2c, 5-ht5, 5-ht6, but the
absence of this action is likely not too dangerous; however: 25x have potentially much higher
intrinsic activity at 5-ht2a than the classical LSD, psilocin, 2C-B, mescaline, comparable instead to DOI, Br-DFLY, etc, and these drugs are indeed more dangerous. LSD is additionally
functionally selective at 5-ht2a, preferring arachidonic acid release over phospholipase C activation; I don't know anything about functional selectivity of other ligands, except that DOI is
not selective.
There is one other possibility: classical psychedelics have an uncharacteristically high therapeutic index (safety) w.r.t. other similarly powerful recreational drugs, and the "safe" drugs seem to consistently have higher 5-ht1a affinity than others. This includes in particular psilocin, mescaline, LSD, DMT, DPT, MDMA, all 4-HO tryptamines, but does
not include any 2C-x, DOx, 25x, etc. 5-ht1a
may play a protective role in the effects of the former class of drugs. In particular are 5-ht1a agonists considered serenic, antipsychotic, and empathogenic; this includes cannabidiol, the anxiolytic buspirone, and the antidepressant/hypnotic trazodone. Indeed, DOM, which was
the first psychedelic to cause really scary overdoses back in 1969, also
lacks any significant 5-ht1a affinity.
In particular, a very intelligent acquaintance had at one point suggested that bronchoconstriction -- which includes shortness of breath and blue lips -- represented a possible risk w.r.t. overdoses on potent 5-ht2a full agonists. I commented at the time that this seemed unlikely, because bronchoconstriction is not seen in serotonin syndrome, which rather includes hyperventilation and hyperthermia (serotonin is a 5-ht2a full agonist obviously), and those in SS do not turn blue.
It appears that I spoke too soon. Turning blue should indicate oxygen depletion (there are other possibilities: methæmoglobinemia). If 5-ht1a is indeed protective, this would explain why users of very high doses of LSD, psilocin, ayahuasca, and mescaline do not have trouble breathing, and why serotonin syndrome does not seem to cause this, but also why Br-DFLY does produce this very undesirable side-effect.
A theory is not complete without errors. The problematic 5-MeO-x tryptamines all have significant affinity at 5-ht1a, which demonstrates that it no guarantor of safety. In one case:
The decedent (male A) was a 29-year-old male. The male A was a sexual partner of male B, who, in search of his [sic] sexual pleasure, injected an aqueous solution of 5-MeO-DIPT into the anus of male A using a dropper in a hotel room at approximately 21:00 h. Following the injection, male A developed abnormal symptoms including very intense agitation and was rushed to hospital. However, he died at about 00:30 h on the following day. Blood and urine specimens were submitted for toxicological examination.
Autopsy findings revealed periarteritis nodosa, involving the heart and liver, a myocardial ischemic lesion, leukocytosis, advanced pulmonary congestion and pulmonary alveolar haemorrhage and periprostatic bleeding.
This is consistent with the inflammatory hypothesis suggested by the nameless one. Lots of lung damage, in fact. Except the last symptom, which seems more likely a result of a condition known as "being fucked in the ass". What is really confusing here is that 5-MeO-DiPT is actually a rather
weak 5-ht2a agonist, instead having
more activity at 5-ht1a.
So there are a couple of acceptable hypotheses:
(1) overdose on NBOMe's causes effects resulting from as-yet-unknown binding to some other, non-psychedelic receptor.
(2) full agonists at 5-ht2a can produce a dangerous inflammatory response, but partial agonists do not; 5-ht1a activity is a red herring
(3) 5-ht1a partially protects from 5-ht2a-mediated inflammatory response and:
(3a) can produce an inflammatory response on its own; there is a necessary, natural "balance" of activity at these receptors
(3b) overdose on 5-MeO-tryptamines results from other, unrelated effects
(4) functional selectivity at 5-ht2a allows LSD, psilocin, et al a wider safety margin: LSD in particular is selective for arachidonic acid release at 5-ht2a over activation of phospholipase C, whereas DOx effects both sorts of activity.
(1) seems unlikely; (2), (3a), (3b), and (4) all seem certainly
possible, but I don't really know enough to prefer any of them over the others.
At this point it would be premature and extremely reckless to assume 5-ht1a agonists can protect from the damaging effects of 25x overdoses.
What I don't know is: are psilocin and mescaline also functionally selective in the manner of LSD? That would be enough to confirm or rule out (4). I also don't know about the intrinsic activity of psilocin at 5-ht2a, which would confirm or rule out (2). LSD is almost a full agonist w.r.t. arachidonic acid, but a weak partial agonist w.r.t. phospholipase C. So (2) is already on shaky ground.
In particular (4) implies that TCB-2, which is selective for phospholipase C activation, could be very dangerous.
(3a) and (3b) are wildcards: if 5-ht1a plays a major role, things get confusing and hard to verify, especially because precisely
what relative activity is safe at 5-ht2a and 5-ht1a is not clear, and finding compounds which have this mix of activity is much harder than finding agonists of a single receptor.
In the meantime: 500 micrograms is probably too high of a starting dose for 25x. The bioavailability of 25x is very inconsistent: they are up to five times as potent by inhalation, and some people may absorb the drugs easier than others, leading to an unexpectedly powerful dose in the bloodstream. To compensate for this, start small, work your way up, and use a consistent dosing method, i.e. don't switch from sublingual to liquid insufflation once you've found a level you like. Caution is advisable for anyone with a history of asthma.
My guess is that vaporization and maybe liquid insufflation have a consistently higher bioavailability than other non-injection methods. I seemed to need less via liquid insufflation than others did sublingually: my use of 25C never exceeded 550 micrograms, and I hit a +3 at 250 micrograms.