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Also question is, which cacti are hallucinogenic?
Mescaline is a psychedelic hallucinogen obtained from the small, spineless cactus Peyote (Lophophora williamsi), the San Pedro cactus, Peruvian torch cactus, and other mescaline-containing cacti.
Correspondingly, are all trichocereus psychoactive?
Some trichocereus cacti are indeed psychoactive, in that they contain mescaline. In 1974 all the Trichocereus species were reclassified into the Echinopsus genus, a move which is still under debate by botanists, because this puts so many plants with wildly different characteristics into the same genus.
The easiest way to determine if your specimen is not a San Pedro, but a toothpick cactus (S. Coryne) is to exam the spines and areoles. These usually come out of VERY enlarged white areoles in sets of 6-8 spines. When wet the spines can appear anywhere from tan to dark-brown and quickly fade to white.