NMDA antagonists like ketamine, PCP, and dextromethorphan can produce subjective effects similar to classic psychedelics.

“So then you have these other classes of compounds that you could call psychedelic. Another big one would be the NMDA antagonist. So this would include ketamine, PCP, and dextromethorphan, something I've done some research with, which folks might recognize from like Robo-tripping, guzzling cough syrup, which is something kind of like high school kids are known to do when they can't get ahold of real drugs, that type of thing. So a large overlap in the types of subjective effects that you get from those compounds compared to the 2A agonist classic psychedelics.”