It's a psychedelic, and in this way it's similar to mushrooms, lsd, ayahuasca, etc.
San Pedro, or T. pachanoi, contains mescaline as main alkaloid.
It's chemically from the phenetylamine family, same as the 2C-x substances or MDMA, but it's more psychedelic and 'smoother' than those IME (and lasts much longer). Low doses mescaline are similar to low dose MDMA, because there is stimulation and empathogenic effects, and no visions (except maybe everything is a bit 'brighter'
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To the OP: Yes, as the attitude section says, we expect you to do some basic research before posting, and there's about a trillion links out there on mescaline and san pedro.