BTW I have a hazy recollection of reading a paper that discussed the various homologues and analogues of methaqualone and it seemed as if ANY para substituent on that pendent aromatic resulted in much higher NMDA activity. I believe THAT is what produces seizures.
Methaqualone binds to something like 23 different subtypes of the GABA receptor (in addition to other classes of receptor). It was an agonist at some sites, a superagonist at some sites (which I suggest it what produces the euphoria) BUT was also an antagonist at some subtypes.
In that paper even the researchers admit that they failed to get a full picture of it's activity and as I've noted, it may very well be that methaqualone and a couple of homologues represent a 'small island of activity' i.e. you have to be very conservative in your choices. You have to actually test the stuff and understand both the acute and chronic toxicity of a novel compound. Get those urine samples and check what metabolites you have. Even then, you need a reasonably large cohort for any such study to be statistically valid. About 130 is a reasonable number.
But even then - you have to accept what is formally known as 'Stage IV trials' AKA pharmacovigilence. Even the minority that passed other tests can and should be yanked from the market if reports suggest bad outcomes may occur.
I know for sure that several potent examples of a class were abandoned because it was recognized that a customer under the influence may redose because their superego is repressed so they make bad choices. If those bad choices result in bad outcomes, it's inherently a bad idea. I was told someone tasted 100mg of pyrazolam and while they slept for a week, they were fine. It was expected, but it was also expected that someone might consume such amounts due to poor decisionmaking and/or as an act of self-harm.
Who knows how the people making carfentanil are able to dissociate from the fact that their product is almost certainly going to lead to very bad outcomes for others. Yes, adults, free will and so forth. I believe in the fifth freedom but I've posted a thread noting that the carfentanil has been catagorized as a WMD. I see little difference between that and openly selling weapons. I sincerely doubt a consumer is in a position to make an informed choice given that it's misrepresented as something else.